UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

Form 10-K

(Mark One)

OR

Commission file number 1-1070

OLIN CORPORATION

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (314) 480-1400

Securities registered pursuant to Section 12(b) of the Act:

Securities registered pursuant to Section 12(g) of the Act: None

Indicate by check mark if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act. Yes    x    No    ¨

Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15(d) of the Exchange Act. Yes    ¨    No    x

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports) and (2) has been subject to such filing requirements for the past 90 days. Yes    x    No    ¨

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K is not contained herein, and will not be contained, to the best of registrant’s knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K.    x

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, or a non-accelerated filer. See definition of “accelerated filer and large accelerated filer” in Rule 12b-2 of the Exchange Act. Large Accelerated Filer x Accelerated Filer ¨ Non-accelerated Filer    ¨

Indicate by check mark whether the registrant is a shell company as defined in Rule 12b-2 of the Exchange Act. Yes    ¨    No    x

As of June 30, 2005, the aggregate market value of registrant’s common stock, par value $1 per share, held by non-affiliates of registrant was approximately $1,295,500,300 based on the closing sale price as reported on the New York Stock Exchange.

As of February 28, 2006, 72,163,828 shares of the registrant’s common stock were outstanding.

DOCUMENTS INCORPORATED BY REFERENCE

Portions of the following document are incorporated by reference in this Form 10-K

as indicated herein:

PART I

Item 1. BUSINESS

GENERAL

Olin Corporation is a Virginia corporation, incorporated in 1892, having its principal executive offices in Clayton, MO. We are a manufacturer concentrated in three business segments: Chlor Alkali Products, Metals and Winchester^®. Chlor Alkali Products manufactures and sells chlorine and caustic soda, sodium hydrosulfite, hydrochloric acid, hydrogen, bleach products and potassium hydroxide, which represent 26% of 2005 sales. Metals products, which represent 59% of 2005 sales, include copper and copper alloy sheet, strip, foil, rod, welded tube, fabricated parts and stainless steel and aluminum strip. Winchester products, which represent 15% of 2005 sales, include sporting ammunition, canister powder, reloading components, small caliber military ammunition and components, and industrial cartridges. See our discussion of our segment disclosures contained in Item 7—“Management’s Discussion and Analysis of Financial Condition and Results of Operations.”

GOVERNANCE

We maintain an Internet website at www.olin.com. Our reports on Form 10-K, Form 10-Q, and Form 8-K, as well as amendments to those reports, are available free of charge on our website, as soon as reasonably practicable after we file the reports with the Securities and Exchange Commission (SEC). Additionally, a copy of our SEC filings can be obtained at the SEC’s Public Reference Room at 450 Fifth Street, N.W., Washington, D.C. 20549 or by calling the SEC at 1-800-SEC-0330. Also, a copy of our electronically filed materials can be obtained at www.sec.gov. Our Principles of Corporate Governance, Committee Charters and Code of Business Conduct are available in the Corporate Governance section of the Investor section of our website at www.olin.com or from the Company by writing to: George Pain, Vice President, General Counsel and Secretary, Olin Corporation, 190 Carondelet Plaza, Suite 1530, Clayton, MO 63105.

In May 2005, our Chief Executive Officer executed the annual Section 303A.12(a) CEO Certification required by the New York Stock Exchange (NYSE), certifying that he was not aware of any violation of the NYSE’s corporate governance listing standards by Olin. Additionally, our Chief Executive Officer and Chief Financial Officer executed the required Sarbanes-Oxley Act of 2002 (SOX) Sections 302 and 906 certifications relating to this Annual Report on Form 10-K, which are filed with the SEC as exhibits to this Annual Report on Form 10-K.

PRODUCTS, SERVICES AND STRATEGIES

Chlor Alkali Products

Products and Services

We have been involved in the U.S. chlor alkali industry for more than 100 years and are a major participant in the U.S. chlor alkali market. Chlorine and caustic soda are co-produced commercially primarily by the electrolysis of salt. These co-products are produced simultaneously, and in a fixed ratio of 1.0 ton of chlorine to 1.1 tons of caustic soda. The industry refers to this as an Electrochemical Unit or ECU. With a demonstrated capacity as of the end of 2005 of 1.24 million ECUs per year, including 50% of the production from our partnership with PolyOne Corporation, which we refer to as our SunBelt joint venture, we are the fourth largest chlor alkali producer in the United States, according to data from Chemical Market Associates, Inc. (CMAI). CMAI is a global petrochemical, plastics and fibers consulting firm established in 1979. According to CMAI data, we are the largest producer measured by production volume of chlorine and caustic soda in the eastern United States, with facilities located in McIntosh, AL, Charleston, TN, Augusta, GA, and Niagara Falls, NY. Since transportation costs can be a significant part of the final cost of the product to the customer, our close proximity to our caustic customers is an advantage. Approximately two-thirds of our caustic soda production is high purity membrane and rayon grade, which according to CMAI data, normally commands a premium selling price in the market.

Our manufacturing facilities in Augusta, McIntosh, Charleston, and a portion of our facility in Niagara Falls are ISO 9002 certified. ISO 9000 (which includes ISO 9001 and ISO 9002) and ISO 14000 (which includes ISO 14001) are sets of related international standards on quality assurance and environmental management developed by the

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International Organization for Standardization to help companies effectively document the quality and environmental management system elements to be implemented to maintain effective quality and environmental management systems. All four of these manufacturing facilities have also achieved Star status in the Voluntary Protection Program (VPP) of the Occupational Safety and Health Administration (OSHA). OSHA’s VPP is a program in which companies voluntarily participate that recognizes facilities for their exemplary safety and health programs. In 2005, Chlor Alkali sought accreditation at its headquarters location and two of its operating plants (Niagara Falls and Charleston) under the RC 14001 Responsible Care^® standard. Official registry board accreditation for all three sites was received by January 18, 2006. RC 14001, which includes the ISO 14001 environmental standard, is a comprehensive integrated management system sanctioned by the chemical industry and recognized by government and regulatory agencies which establishes requirements for the management of the safety, health, environmental, security, transportation, product stewardship, and stakeholder engagement activities of the business. Accreditation at the two remaining operating facilities (Augusta and McIntosh) is scheduled to be completed in 2006.

Chlorine is used as a raw material in the production of thousands of products, but a significant portion of U.S. chlorine production is consumed in the manufacture of ethylene dichloride, or EDC, a precursor for polyvinyl chloride, or PVC. PVC is a plastic used in applications such as vinyl siding, plumbing and automotive parts. Other U.S. end-uses for chlorine include chlorinated intermediates, isocyanates and water treatment. While much of the chlorine produced in the U.S. is consumed by the producing company to make downstream products, we sell most of the chlorine we produce to third parties in the merchant market.

Caustic soda has a wide variety of end use applications, the largest of which is in the pulp and paper industry. Caustic soda is also used in the production of detergents and soaps, alumina and a variety of other inorganic and organic chemicals.

The chlor alkali industry is cyclical, both as a result of changes in demand for each of the co-products and as a result of the large increments in which new capacity is added. Because chlorine and caustic are produced in a fixed ratio, the supply of one product can be constrained both by the physical capacity of the production facilities and/or by the ability to sell the co-product. Prices for both products respond rapidly to changes in supply and demand. Our ECU netbacks (defined as gross selling price less freight and discounts) averaged approximately $300 per ECU in the first half of 2004 and then increased quarter over quarter through 2005, with fourth quarter 2005 netbacks averaging approximately $545 per ECU.

Electricity and salt are the major purchased raw materials for our Chlor Alkali Products segment. Raw materials represent approximately 50% of the total cost of producing an ECU. Electricity is the single largest raw material component in the production of chlor alkali products. During the past two years, we experienced an increase in the cost of electricity from our suppliers due to price increases in natural gas and coal. We are supplied by utilities that primarily utilize coal, hydroelectric and nuclear power and have relatively minor exposure to natural gas. The majority of the salt used in our Chlor Alkali Products segment is produced from internal resources but we do purchase salt in the merchant market. The commodity nature of this industry places an added emphasis on cost management and we believe that we have managed our manufacturing costs in a manner that makes us one of the low cost producers in the industry. In addition, as market demand grows in the future, we believe the design of the SunBelt joint venture plant will enable us to expand capacity cost-effectively.

We also manufacture and sell other chlor alkali-related products and we recently invested in capacity and product upgrades in some of these areas. These products include chemically processed salt, hydrochloric acid, sodium hypochlorite, hydrogen, and potassium hydroxide. We also sell sodium hydrosulfite to paper, textile and clay bleaching customers.

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The following table lists products of our Chlor Alkali Products business, with principal products on the basis of annual sales highlighted in bold face.

Strategies

Continued Role as a Preferred Supplier to Merchant Market Customers.    Based on our market research, we believe our Chlor Alkali Products business is viewed as a preferred supplier by our merchant market customers. We will continue to focus on providing quality customer service support and developing relationships with our valued customers.

Pursue Incremental Expansion Opportunities.    We have invested in capacity and product upgrades in our chemically processed salt, hydrochloric acid, sodium hypochlorite, potassium hydroxide and hydrogen businesses. These expansions increase our captive use of chlorine while increasing the sales of these co-products. These niche businesses provide opportunities to upgrade chlorine and caustic to higher value-added applications. We also have the opportunity, when business conditions permit, to pursue incremental expansion through our SunBelt joint venture.

Metals

Products and Services

We have been in the Metals business for 89 years. Based on Copper Development Association Inc. (CDA) data, we are a leading manufacturer of copper and copper alloy sheet, strip, plate, foil and brass rod in the United States. CDA acts as the central authoritative source of data and information pertaining to the U.S. copper and brass industry. While primarily processing copper and copper alloys, we also reroll and form other metals, such as aluminum and stainless steel. We believe we hold leading positions for premium priced, high performance alloys in the United States. We also supply high performance alloys to non-U.S. customers through exports, technology licensing, joint ventures and local distribution. Participants in the copper and copper alloy sheet and strip industry include integrated mills, reroll mills and distributors, with many participants engaging in multiple roles. We believe that we are the largest U.S. participant in each of these categories. We believe that our status as the largest U.S. participant affords us a favorable industry position. We also believe we are one of the lowest cost producers, a quality and service leader and a specialty product innovator.

All of our copper and copper alloy sheet and strip mills are both QS 9000 and ISO 9000 certified. QS 9000 is an international automotive standard developed by General Motors, Ford Motor Company and Chrysler to harmonize the fundamental supplier quality systems as an assessment tool, and is based upon ISO 9000 standards. All sheet and strip and rod locations are ISO certified. Our brass rod mill is ISO 9002 certified.

We maintain advantages over our competition through our patent-protected technologies. We believe our high performance alloys provide superior strength, conductivity and formability to customers in the automotive, electrical, electronic and

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telecommunications industries. We currently hold 37 U.S. patents associated with high performance alloys and 39 other U.S. patents related to various proprietary processing and technical capabilities, many of which are also registered in foreign jurisdictions. To further our global presence, we established a joint venture with Yamaha Corporation in Japan to produce high performance alloys, formed a technical alliance with Wieland-Werke A.G. of Germany under which we jointly develop new high performance alloys and participate in an alloy licensing arrangement and formed a joint venture with Luoyang Copper (Group) Ltd. in China to operate a metals service center to supply the growing Chinese demand. These relationships provide us with greater global reach and enable us to provide high performance alloys in Asia and Europe.

In addition, through sales of our clad metal, produced by a proprietary cladding process, we believe we are a major supplier of coinage metal to the U.S. Mint. We also supply coinage metal to other world governments. Our Metals segment produces ammunition cartridge cups for use captively in the manufacture of our Winchester sporting ammunition, which constitutes a small portion of our total Metals segment output. We also sell cartridge brass to other ammunition makers. This relationship with Winchester, along with our fabrication business, provides us with a significant captive customer base.

Brass and other copper alloys are manufactured by melting copper together with various combinations of zinc, lead or other metals. The resulting product goes through a series of processes, including casting, hot rolling, milling, cold rolling, annealing, cleaning and slitting to produce sheet and strip. A similar process is followed for the production of rod. The principal end-uses for sheet and strip products include: automotive (connectors and radiators); electronics (lead frames, connectors, wiring and telecommunications applications); ammunition; coinage; and other applications such as builder’s hardware, plumbing supplies and welded tube for utility condensers and industrial heat exchangers. Brass rod is used to produce a variety of products, such as faucets, plumbing fittings, heating and air conditioning components, industrial valves, automotive parts and numerous hardware components.

The major raw materials used in our metals business are brass scrap, copper, zinc, and other non-ferrous metals, purchased from merchants, dealers, and customers at market prices.

Historically, demand for copper and copper alloy sheet and strip and rod has exhibited growth consistent with the growth in the U.S. gross domestic product. In the late 1990’s and in 2000, demand expanded at a rapid pace principally due to the strength in the electronics segment and the introduction of the state quarter program by the U.S. Mint. From 1997 to 2000, sheet and strip demand grew at an annualized growth rate of 8%. Since 2001, strip and rod demand has been unchanged at levels approximately 10% and 20%, respectively, below that of the late 1990’s.

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The following table lists products and services of our Metals business, with principal products highlighted in bold face.

Strategies

Continue Profitable Growth Globally.    Our goal is to be a leading worldwide supplier of specialty copper-based products and related engineered materials. We intend to achieve this goal by building our high performance alloys business on a global basis. In 2004 and continuing into 2005, we took a number of steps to continue to grow our global presence including the start-up of our joint venture service center in Guangzhou, China, the development of new business opportunities in Europe and the acquisition and integration of the aluminum strip distribution assets of Metal Foils LLC in the United States.

Maintain Premier Specialty Product Innovator Position.    We believe that we manufacture more high performance alloys than any other competitor, and we continue to allocate resources to maximize this product line. Our specialty products include proprietary high performance alloys and materials that meet strength, gauge, formability, and conductivity requirements for applications in our customers’ industries.

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Increase Cost Efficiencies.    We plan to continue to focus on achieving economies of scale, improved manufacturing processes, and innovation in pursuit of cost reductions. We strive for profit improvements primarily through yield improvements, increased equipment utilization and capacity enhancements.

Continue Our Quality Leadership.    We maintain ISO 9000, QS 9000, and ISO 14001 certifications. For example, our East Alton, IL mill carries the distinctive certifications of ISO 9001, due to its extensive design work, and ISO 14001, a prominent environmental standard. We believe that these certifications demonstrate a quality advantage not possessed by our key U.S. competitors. We also continue to maintain preferred supplier positions with some of the largest or most respected companies in segments where quality is essential, such as automotive and electronics.

Leverage Our Service and Distribution Leadership for Growth.    We believe that we are a service and distribution leader in the copper-based metals industry. Our A.J. Oster distribution system extends throughout the United States and also includes facilities in Puerto Rico and Mexico. We sell directly from the mill to large volume customers, and to small and medium size customers through A.J. Oster and other licensed distributors. We intend to leverage our service leadership and our distribution network to improve our just-in-time delivery services and our customized order capabilities.

Winchester

Products and Services

Winchester is in its 139^th year of operation and its 75^th year as part of Olin. Winchester is a premier developer and manufacturer of small caliber ammunition for sale to domestic and international retailers, law enforcement agencies and domestic and international militaries. We believe we are a leading U.S. producer of ammunition for recreational shooters, hunters, law enforcement agencies and the U.S. Armed Forces. Our legendary Winchester product line includes all major gauges and calibers of shotgun shells, rimfire and centerfire ammunition for pistols and rifles, canister powder, reloading components and industrial cartridges. We believe we are the leading U.S. supplier of small caliber commercial ammunition.

Winchester has strong relationships throughout the sales and distribution chain and strong ties to traditional dealers and distributors. Winchester has built its business with key high volume mass merchants and specialty sporting goods retailers. We have consistently developed industry-leading ammunition, and for nine of the last fourteen years, Winchester was recognized with the “Ammunition of the Year” award from the Shooting Industry Academy of Excellence for its technological and design leadership.

Winchester purchases raw materials such as lead from vendors at market prices as posted on exchanges such as the Commodity Exchange, or COMEX, and London Metals Exchange, or LME. Winchester also purchases copper-based strip and cups from our Metals segment. Winchester’s other main raw material is propellant, which is purchased predominantly from one of the United States’ largest propellant suppliers.

The following table lists products and services of our Winchester business, with principal products on the basis of annual sales highlighted in bold face.

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Strategies

Leverage Existing Strengths.    Winchester plans to focus on seeking new opportunities to leverage the legendary Winchester brand name and will continue to offer a full line of ammunition products to the markets we serve, with specific focus on investments that lower our costs and that make Winchester ammunition the retail brand of choice.

Focus on Product Line Growth.    With a long record of pioneering new product offerings, Winchester has built a strong reputation as an industry innovator. This includes the introduction of reduced-lead and non-lead products, which are growing in popularity for use in indoor shooting ranges and for outdoor hunting.

INTERNATIONAL OPERATIONS

We have sales offices and subsidiaries in various countries which support the worldwide export of products from the United States as well as overseas production facilities.

Yamaha-Olin Metal Corporation, of which we are a 50% owner, manufactures high-performance copper alloys in Japan for sale to the electronics industry throughout the Far East. Our subsidiary, Olin Australia Limited, loads and packs sporting and industrial ammunition in Australia. We entered into an agreement with Luoyang Copper (Group) Ltd. to jointly construct and operate a metals service center in Guangzhou, China, which became operational in the first quarter of 2004. See the Note “Segment Information” of the Notes to Consolidated Financial Statements in Item 8, for geographic segment data. We are incorporating our segment information from that Note into this section of our Form 10-K.

CUSTOMERS AND DISTRIBUTION

During 2005, no single customer accounted for more than 4% of consolidated sales. Sales to all U.S. government agencies and sales under U.S. government contracting activities in total accounted for approximately 10% of consolidated sales in 2005. Products we sell to industrial or commercial users or distributors for use in the production of other products constitute a major part of our total sales. We sell some of our products, such as caustic soda, sporting ammunition, and metals products, to a large number of users or distributors, while we sell others, such as chlorine, in substantial quantities to a relatively small number of industrial users. We discuss the customers for each of our three businesses in more detail above under “Products and Services.”

We market most of our products and services primarily through our sales force and sell directly to various industrial customers, the U.S. Government and its prime contractors, to wholesalers and other distributors.

Because we engage in some government contracting activities and make sales to the U.S. Government, we are subject to extensive and complex U.S. Government procurement laws and regulations. These laws and regulations provide for ongoing government audits and reviews of contract procurement, performance and administration. Failure to comply, even inadvertently, with these laws and regulations and with laws governing the export of munitions and other controlled products and commodities could subject us or one or more of our businesses to civil and criminal penalties, and under certain circumstances, suspension and debarment from future government contracts and the exporting of products for a specified period of time.

COMPETITION

We are in active competition with businesses producing the same or similar products, as well as, in some instances, with businesses producing different products designed for the same uses. We are among the largest manufacturers or distributors in the United States of ammunition, copper based strip, rod, and certain chlor alkali products based on data provided by the Sporting Arms and Ammunition Manufacturers’ Institute (SAAMI), CDA and CMAI, respectively. Founded in 1926, SAAMI is an association of the nation’s leading manufacturers of sporting firearms, ammunition and components. Many factors influence our ability to compete successfully, including price, delivery, service, performance, product innovation and product recognition and quality, depending on the product involved.

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EMPLOYEES

As of December 31, 2005, we had approximately 5,900 employees, with 5,800 working in the United States and 100 working in foreign countries. Various labor unions represent a majority of our hourly-paid employees for collective bargaining purposes.

During the fourth quarter of 2005, we reached a three-year agreement with all five labor unions representing approximately 3,000 employees at our East Alton facility where both Metals and Winchester are headquartered.

A description of the labor contracts that are due to expire in the near future are listed below:

While we believe our relations with our employees and their various representatives are generally satisfactory, we cannot assure that we can conclude these labor contracts or any other labor agreements without work stoppages and cannot assure that any work stoppages will not have a material adverse effect on our business, financial condition or results of operations.

RESEARCH ACTIVITIES; PATENTS

Our research activities are conducted on a product-group basis at a number of facilities. Company-sponsored research expenditures were $4.2 million in 2005, $3.9 million in 2004, and $4.7 million in 2003.

We own or license a number of patents, patent applications and trade secrets covering our products and processes, particularly for use in our Metals segment. We believe that, in the aggregate, the rights under our patents and licenses are important to our operations, but we do not consider any individual patent or license or group of patents and licenses related to a specific process or product to be of material importance to our total business.

RAW MATERIALS AND ENERGY

We purchase the major portion of our raw material requirements. The principal basic raw materials for our production of chlor alkali products are salt, electricity, sulfur dioxide, and hydrogen. The majority of the salt used in our Chlor Alkali Products segment is produced from internal resources. Copper, zinc, various other nonferrous metals and brass scrap are required for the Metals business. Lead, brass, and propellant are the principal raw materials used in the Winchester business. We typically purchase our electricity, salt, sulfur dioxide, and propellants pursuant to multi-year contracts. In the manufacture of ammunition, we use a substantial percentage of our own output of cartridge brass. We provide additional information with respect to specific raw materials in the tables above under “Products and Services.”

Electricity is the predominant energy source for our manufacturing facilities. Most of our facilities are served by utilities which generate electricity principally from coal, hydroelectric and nuclear power.

ENVIRONMENTAL AND TOXIC SUBSTANCES CONTROLS

The establishment and implementation of federal, state and local standards to regulate air, water and land quality have affected and will continue to affect substantially all of our manufacturing locations. Federal legislation providing for regulation of the manufacture, transportation, use and disposal of hazardous and toxic substances has imposed additional regulatory requirements on industry, particularly the chemicals industry. In addition, implementation of environmental laws, such as the Resource Conservation and Recovery Act and the Clean Air Act, has required and will continue to require new capital expenditures and will increase operating costs. We employ waste minimization and pollution prevention programs at our manufacturing sites and we are a party to various governmental and private environmental actions associated with waste disposal sites and manufacturing facilities. Charges or credits to income for investigatory and remedial efforts were material to operating results in the past three years and may be material to net income in future years.

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See our discussion of our environmental matters in Item 3, “Legal Proceedings” below, the Note “Environmental” of the Notes to Consolidated Financial Statements contained in Item 8, and Item 7, “Management’s Discussion and Analysis of Financial Condition and Results of Operations.”

Item 1A. RISK FACTORS

In addition to the other information in this Form 10-K, the following factors should be considered in evaluating Olin and our business. All of our forward-looking statements should be considered in light of these factors. Additional risks and uncertainties that we are unaware of or that we currently deem immaterial also may become important factors that affect us.

Sensitivity to Global Economic Conditions and Cyclicality—Our operating results could be negatively affected during economic downturns.

The business of most of our customers, particularly our vinyl, urethanes, pulp and paper, automotive, coinage, electrical connectors, telecommunications and housing customers, are, to varying degrees, cyclical and have historically experienced periodic downturns. These economic and industry downturns have been characterized by diminished product demand, excess manufacturing capacity and, in some cases, lower average selling prices. Therefore, any significant downturn in our customers’ businesses or in global economic conditions could result in a reduction in demand for our products and could adversely affect our results of operations or financial condition. As a result of the depressed economic conditions beginning in the fourth quarter of 2000 and continuing through the first half of 2002, our vinyls, urethanes and pulp and paper customers had lower demand for our chlor alkali products. During the period 2000-2003, demand for Chlor Alkali products was low enough to lead to plant shutdowns within the industry and about 12% of capacity was removed from North American production. When demand improved in early 2004, the operating rates increased to the mid to high 90% range, resulting in a tight supply/demand balance which has resulted in increasing pricing. We believe no new significant capacity is anticipated to come on stream until late 2007 or early 2008, and as a result, we believe supply/demand is expected to remain relatively tight and pricing is expected to remain above historical levels. Lower demand in our Metals segment has adversely affected our business and results of operations since 2001. The rod industry has been negatively affected by customers’ migration to lower-cost, offshore locations and by continued reductions in capital spending in the industrial machinery segment and reduced demand for building and household products as a result of declines in commercial construction. In 2005, Metals U.S. demand was down approximately 8% from 2004. Metals demand in the automotive and building products segments were down in 2005, while ammunition, coinage, and electronics market segment shipments all experienced increases over 2004.

Although we do not generally sell a large percentage of our products directly to customers abroad, a large part of our financial performance is dependent upon a healthy economy beyond the United States. Our customers sell their products abroad. As a result, our business is affected by general economic conditions and other factors in Western Europe and most of East Asia, particularly China and Japan, including fluctuations in interest rates, customer demand, labor costs and other factors beyond our control. The demand for our customers’ products, and therefore, our products, is directly affected by such fluctuations. We cannot assure you that events having an adverse effect on the industries in which we operate will not occur or continue, such as a further downturn in the Western European, Asian or world economies, increases in interest rates, unfavorable currency fluctuations or a prolonged slowdown in the coinage, electronic or telecommunications industries.

The terrorist attacks of September 11th created many economic and political uncertainties and have had a negative impact on the global economy. The long-term effects of these attacks on our future operating results and financial condition are unknown. The national and international responses to terrorist attacks and the potential for additional terrorist attacks or similar events could have further material adverse effects on the economy in general, on our industry and on our operations. For example, war with one or more countries could have numerous consequences for us and our customers, one of which may be sustained high energy prices.

Cyclical Pricing Pressure—Our profitability could be reduced by declines in average selling prices of our products, particularly declines in the ECU netback for chlorine and caustic.

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Our historical operating results reflect the cyclical and sometimes volatile nature of the chemical, metals and ammunition industries. We experience cycles of fluctuating supply and demand in each of our business segments, particularly in Chlor Alkali Products, which results in changes in selling prices. Periods of high demand, tight supply and increasing operating margins tend to result in increased capacity and production until supply exceeds demand, generally followed by periods of oversupply and declining prices. The industry build cycle, and its impact on industry pricing, has been most pronounced in our Chlor Alkali Products segment. For example, in 1995 and 1996, the chlor alkali industry was very profitable due to a tight supply/demand balance, which resulted in both higher operating rates and higher ECU prices. Higher profits led to reinvestment to expand capacity. This new capacity became operational in 1998 and 1999, resulting in industry over-capacity. This imbalance was exacerbated by falling demand as a result of the Asian financial crisis. The supply/demand imbalance resulted in both lower operating rates and lower ECU prices, and in 1999, many chlor alkali producers had operating losses. The supply/demand balance improved due to improved economic conditions in 2000 compared to 1999, and ECU prices increased in 2000 compared to 1999. As the U.S. and world economies deteriorated in 2001 and through the first half of 2002, the chlor alkali industry again experienced a period of oversupply because of lower industry demand for both chlorine and caustic. During the 2000-2003 timeframe about 12% of North American production was shut down which caused operating rates to improve without much improvement in demand. In late 2003 and early 2004, chlorine demand began to strengthen and operating rates increased to the mid to high 90% range. Caustic demand began to strengthen by mid year 2004 and supplies of both products have been tight since that time. This has resulted in price increase initiatives over the last 18 months. We believe that with supply and demand in balance, and no new capacity anticipated to be available in the next couple of years, 2005/2006 may be the cycle peak for the Chlor Alkali Industry. Another factor impacting demand for chlorine and caustic soda is the price of natural gas. Higher natural gas prices, which through 2005 have exceeded $10 per million British thermal units, increase our customers’ manufacturing costs, and depending on the crude oil to gas ratio, could make them less competitive in world markets and, therefore, may result in reduced demand for our products.

Price in the chlor alkali industry is a major supplier selection criterion. We have little or no ability to influence prices in this large commodity market. Decreases in the average selling prices of our products could have a material adverse effect on our profitability. For example, assuming all other costs remain constant and internal consumption remains approximately the same, a $10 per ECU selling price change equates to an approximate $11 million annual change in our revenues and pretax profit when we are operating at full capacity. While we strive to maintain or increase our profitability by reducing costs through improving production efficiency, emphasizing higher margin products, and by controlling transportation, selling, and administration expenses, we cannot assure you that these efforts will be sufficient to offset fully the effect of changes in pricing on operating results.

Because of the cyclical nature of our businesses, we cannot assure you that pricing or profitability in the future will be comparable to any particular historical period, including the most recent period shown in our operating results. We cannot assure you that the chlor alkali industry will not experience adverse trends in the future, or that our operating results and/or financial condition will not be adversely affected by them.

Our Metals and Winchester segments are also subject to changes in operating results as a result of cyclical pricing pressures, but to a lesser extent than the Chlor Alkali Products segment. We generally pass changes in prices for copper and other metals along to our customers as part of the negotiated price of the finished product in most of our Metals segment product lines. However, our Metals segment experiences pricing pressure with respect to its conversion charges, and we cannot assure you that adverse trends in pricing and margins will not affect operating results in the future. Changes in global supply/demand for copper and copper alloys may affect our ability to obtain raw materials under reasonable terms and conditions which may materially adversely affect our operating results. Similarly, selling prices of ammunition are affected by changes in raw material costs and availability and customer demand, and declines in average selling prices of our Winchester segment could adversely affect our profitability.

Imbalance in Demand for Our Chlor Alkali Products—A loss of a substantial customer for our chlorine or caustic soda could cause an imbalance in demand for these products, which could have an adverse effect on our results of operations.

Chlorine and caustic soda are produced simultaneously and in a fixed ratio of 1.0 ton of chlorine to 1.1 tons of caustic soda. The loss of a substantial chlorine or caustic soda customer could cause an imbalance in demand for our

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chlorine and caustic soda products. An imbalance in demand may require us to reduce production of both chlorine and caustic soda or take other steps to correct the imbalance. Since we cannot store chlorine, we may not be able to respond to an imbalance in demand for these products as quickly or efficiently as some of our competitors. If a substantial imbalance occurred, we would need to reduce prices or take other actions that could have a negative impact on our results of operations and financial condition.

Pension Plans—The impact of declines in global equity markets on asset values and any declines in interest rates used to value the liabilities in our pension plan may result in higher pension costs and the need to fund the pension plan in future years in material amounts.

Under Statement of Financial Accounting Standards (SFAS) No. 87, “Employers’ Accounting for Pensions,” we recorded non-cash after-tax charges of $29.2 million ($47.8 million pretax) and $23.7 million ($38.8 million pretax) to Shareholders’ Equity as of December 31, 2005 and December 31, 2004, respectively. These charges reflect an accumulated benefit obligation in excess of the year-end market value of assets of our pension plan. In 2005, the cost impact of plan changes and the adverse effect of lower interest rates more than offset the increase in the value of plan assets and in 2004, the adverse impact of lower interest rates more than offset the increase in plan assets, which necessitated the recording of these after-tax charges to shareholders’ equity. These are non-cash charges and do not affect our ability to borrow under our revolving credit agreement.

The determinations of pension expense and pension funding are based on a variety of rules and regulations. Changes in these rules and regulations could impact the calculation of pension plan liabilities and the valuation of pension plan assets. They may also result in higher pension costs, additional financial statement disclosure, and accelerate and increase the need to fund the pension plan. There are currently a number of legislative proposals being considered, that if enacted, would change the current rules. Most of these proposals would accelerate the pension funding compared to funding under the existing rules.

In addition, the impact of declines in global equity and bond markets on asset values may result in higher pension costs and may increase and accelerate the need to fund the pension in future years. For example, holding all other assumptions constant, a one hundred basis point decrease or increase in the assumed rate of return on plan assets would have increased or decreased, respectively, the 2005 pension cost by approximately $12.9 million.

Holding all other assumptions constant, a 50 basis point decrease or increase in the discount rate used to calculate pension costs for 2005 would have increased or decreased pension costs by $5.2 million and $6.3 million, respectively. The same 50 basis point increase or decrease in the discount rate would have increased or decreased the accumulated benefit obligation by $84.0 million.

Environmental Costs—We have ongoing environmental costs, which could materially adversely affect our financial position or results of operations.

The nature of our operations and products, including the raw materials we handle, exposes us to the risk of liabilities or claims with respect to environmental matters. We have incurred, and expect to incur, significant costs and capital expenditures in complying with environmental laws and regulations.

The ultimate costs and timing of environmental liabilities are difficult to predict. Liability under environmental laws relating to contaminated sites can be imposed retroactively and on a joint and several basis. One liable party could be held responsible for all costs at a site, regardless of fault, percentage of contribution to the site or the legality of the original disposal. We could incur significant costs, including cleanup costs, natural resources damages, civil or criminal fines and sanctions and third-party lawsuits claiming, for example, personal injury and/or property damage, as a result of past or future violations of, or liabilities under, environmental or other laws.

In addition, future events, such as changes to or more rigorous enforcement of environmental laws, could require us to make additional expenditures, modify or curtail our operations and/or install pollution control equipment.

Accordingly, it is possible that some of the matters in which we are involved or may become involved may be resolved unfavorably to us, which could materially adversely affect our financial position or results of operations. See “Management’s Discussion and Analysis of Financial Condition and Results of Operations-Environmental Matters.”

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Competition—We face competition from other chemical, metals and ammunition companies, including the migration by United States customers to low-cost foreign locations, which could adversely affect our revenues and financial condition.

We are in active competition with companies producing the same or similar products, as well as, in some instances, with companies producing different products designed for the same uses. With respect to certain product groups, such as ammunition, copper alloys and brass rod, and with respect to certain chlor alkali products, we are among the largest manufacturers or distributors in the United States. We encounter competition in price, delivery, service, securing and maintaining customers, performance, technology, product innovation, and product recognition and quality, depending on the product involved. Our customers could decide to move some or all of their production to lower cost, offshore locations and this could reduce demand in the United States for our products. With respect to certain products, some of our competitors are larger, have greater financial resources and have less debt than we do. As a result, these competitors may be better able to withstand a change in conditions within the industries in which we operate and throughout the economy as a whole. If we do not compete successfully, our business, financial condition and results of operations could be adversely affected.

Cost Control—Our profitability could be reduced if we experience higher-than-expected raw material, utility, transportation or logistics costs, or if we fail to achieve our targeted cost reductions.

Our operating results and profitability are dependent upon our continued ability to control, and in some cases further reduce, our costs. If we are unable to do so, or if costs outside of our control, particularly our costs of raw materials, utilities, transportation and similar costs, increase beyond anticipated levels, our profitability will decline.

Production Hazards—Our facilities are subject to operating hazards, which may disrupt our business.

We are dependent upon the continued safe operation of our production facilities. Our production facilities are subject to hazards associated with the manufacture, handling, storage and transportation of chemical materials and products and ammunition, including leaks and ruptures, explosions, fires, inclement weather and natural disasters, unexpected utility disruptions or outages, unscheduled downtime and environmental hazards. From time to time in the past, we have had incidents that have temporarily shut down or otherwise disrupted our manufacturing, causing production delays and resulting in liability for workplace injuries and fatalities. Some of our products involve the manufacture and/or handling of a variety of explosive and flammable materials. Use of these products by our customers could also result in liability if an explosion, fire, spill or other accident were to occur. We cannot assure you that we will not experience these types of incidents in the future or that these incidents will not result in production delays or otherwise have a material adverse effect on our business, financial condition or results of operations.

Litigation and Claims—We are subject to litigation and other claims, which could cause us to incur significant expenses.

We are a defendant in a number of pending legal proceedings relating to our present and former operations. These include proceedings alleging injurious exposure of plaintiffs to various chemicals and other substances (including proceedings based on alleged exposures to asbestos). Frequently, such proceedings involve claims made by numerous plaintiffs against many defendants. We believe we have valid defenses to these proceedings and are defending them vigorously. However, because of the inherent uncertainties of litigation, we are unable to predict the outcome of these proceedings and therefore cannot determine whether the financial impact, if any, will be material to our financial position or results of operations.

Indebtedness—Our indebtedness could adversely affect our financial condition and limit our ability to grow and compete, which could prevent us from fulfilling our obligations under our indebtedness.

As of December 31, 2005, we had $258.3 million of indebtedness outstanding, including $6.9 million representing the fair value related to $131.6 million of interest rate swaps in effect at December 31, 2005 and excluding our guarantee of $73.1 million of indebtedness of our SunBelt joint venture. This does not include our $160 million senior credit facility on which we had $122.9 million available on that date because we had issued $37.1 million of letters of credit. As of December 31, 2005, our indebtedness represented 38% of our total capitalization.

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Our indebtedness could adversely affect our financial condition and limit our ability to grow and compete, which in turn could prevent us from fulfilling our obligations under our indebtedness. Despite our level of indebtedness, the terms of our senior credit facility and our existing indentures permit us to borrow additional money. If we borrow more money, the risks related to our indebtedness could increase significantly.

Debt Service—We may not be able to generate sufficient cash to service our debt, which may require us to refinance our indebtedness or default on our scheduled debt payments.

Our ability to generate sufficient cash flow from operations to make scheduled payments on our debt depends on a range of economic, competitive and business factors, many of which are outside our control. We cannot assure you that our business will generate sufficient cash flow from operations. If we are unable to meet our expenses and debt obligations, we may need to refinance all or a portion of our indebtedness on or before maturity, sell assets or raise equity. We cannot assure you that we would be able to refinance any of our indebtedness, sell assets or raise equity on commercially reasonable terms or at all, which could cause us to default on our obligations and impair our liquidity. Our inability to generate sufficient cash flow to satisfy our debt obligations, or to refinance our obligations on commercially reasonable terms, would have an adverse effect on our business, financial condition and results of operations, as well as on our ability to satisfy our debt obligations. See “Management’s Discussion and Analysis of Financial Condition and Results of Operations.”

At December 31, 2005, we had interest rate swaps of $131.6 million, which convert a portion of our fixed rate debt to a variable rate. As a result, 54% of our indebtedness bears interest at variable rates that are linked to short-term interest rates. If interest rates rise, our costs relative to those obligations would also rise.

Tax Audits—We are currently subject to ongoing tax audits, which may result in additional tax payments.

During 2005, the Internal Revenue Service (IRS) substantially completed its audit of our Federal income tax return for the years 2001 and 2002. We are currently contesting various issues before the Appeals Division of the IRS with respect to the years 1996 through 2002. Depending on the outcome of these audits, we may be required to pay additional taxes, and any additional taxes and related interest could be substantial. We have reserved amounts which we believe will be sufficient for any adverse outcome. The timing of any such payments is uncertain.

Labor Matters—We cannot assure you that we can conclude future labor contracts or any other labor agreements without work stoppages.

Various labor unions represent a majority of our hourly-paid employees for collective bargaining purposes. A description of the labor contracts that are due to expire in the near future are listed below:

While we believe our relations with our employees and their various representatives are generally satisfactory, we cannot assure that we can conclude future labor contracts or any other labor agreements without work stoppages and cannot assure that any work stoppages will not have a material adverse effect on our business, financial condition or results of operations.

Security and Chemicals Transportation—New regulations on the transportation of hazardous chemicals and/or the security of chemical manufacturing facilities and public policy changes related to transportation safety could result in significantly higher operating costs.

The chemical industry, including the chlor alkali industry, has proactively responded to the issues surrounding the events of September 11, 2001 by starting new initiatives relating to the security of chemicals industry facilities and the transportation of hazardous chemicals in the United States. Simultaneously, government at the local, state, and federal levels has begun the regulatory process which could lead to new regulations that would impact the security

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of chemical plant locations and the transportation of hazardous chemicals. Our Chlor Alkali business could be adversely impacted because of an incident at one of our facilities or an incident while transporting product or the cost of complying with new regulations. The extent of the impact would depend on the consequences of an incident and the nature and direction of future regulations, which are unknown at this time.

Changes in Laws and Regulations—We are subject to a variety of existing laws and regulations that affect our business.

We are unable to determine what effect, if any, the impact of changes in existing or new laws and regulations and the associated compliance costs may have on our operating results.

Item 1B. UNRESOLVED STAFF COMMENTS

Not applicable.

Item 2. PROPERTIES

We have manufacturing sites at 21 separate locations in 12 states and Puerto Rico, and two manufacturing sites and two metal service centers in four foreign countries, including the Yamaha-Olin Metal Corporation joint venture facility. The metals service center in China became operational in the first quarter of 2004. Most manufacturing sites are owned although a number of small sites are leased. We listed the locations at or from which our products and services are manufactured, distributed, or marketed in the tables set forth under the caption “Products and Services.”

We lease warehouses, terminals and distribution offices and space for executive and branch sales offices and service departments throughout the world.

Item 3. LEGAL PROCEEDINGS

(a) We have completed all work in connection with remediation of mercury contamination at the site of our former mercury cell chlor alkali plant in Saltville, VA required to date. In mid-2003, the Trustees for natural resources in the North Fork Holston River, the Main Stem Holston River, and associated floodplains, located in Smyth and Washington Counties in Virginia and in Sullivan and Hawkins Counties in Tennessee notified us of, and invited our participation in, an assessment of alleged injuries to natural resources resulting from the release of mercury. The Trustees also notified us that they have made a preliminary determination that we are potentially liable for natural resource damages in said rivers and floodplains. We have agreed to participate in the assessment. In light of the early stage, and inherent uncertainties, of the assessment, we cannot at this time determine whether the financial impact, if any, of this matter will be material to our financial position or results of operations. See “Environmental Matters” contained in Item 7—“Management’s Discussion and Analysis of Financial Condition and Results of Operations.”

(b) As part of the continuing environmental investigation by federal, state and local governments of waste disposal sites, we have entered into a number of settlement agreements requiring us to participate in the investigation and cleanup of a number of sites. Under the terms of such settlements and related agreements, we may be required to manage or perform one or more elements of a site cleanup, or to manage the entire remediation activity for a number of parties, and subsequently seek recovery of some or all of such costs from other Potentially Responsible Parties (PRPs). In many cases, we do not know the ultimate costs of our settlement obligations at the time of entering into particular settlement agreements, and our liability accruals for our obligations under those agreements are often subject to significant management judgment on an ongoing basis. Those cost accruals are provided for in accordance with generally accepted accounting principles and our accounting policies set forth in the environmental matters section in Item 7—Management’s Discussion and Analysis of Financial Condition and Results of Operations.

(c) We and our subsidiaries are defendants in various other legal actions (including proceedings based on alleged exposures to asbestos) incidental to our past and current business activities. While we believe that none of these legal actions will materially impact our financial position, in light of the inherent uncertainties of the litigation concerning alleged exposures, we cannot at this time determine whether the financial impact, if any, of these matters will be material to our results of operations.

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We have been named as defendant in a number of similar legal actions now pending in federal court in San Jose, CA relating to alleged groundwater contamination arising from perchlorate use between 1956 and 1996 by Olin and another unrelated defendant at an Olin facility in Morgan Hill, CA. We have settled with virtually all of the plaintiffs in these legal actions and continue working with California state regulatory authorities to determine the scope of potential contamination.

Item 4. SUBMISSION OF MATTERS TO A VOTE OF SECURITY HOLDERS

We did not submit any matter to a vote of security holders during the three months ended December 31, 2005.

Executive Officers as of February 28, 2006

No family relationship exists between any of the above named executive officers or between any of them and any of our directors. Such officers were elected to serve, subject to the By-laws, until their respective successors are chosen.

J. D. Rupp and J. L. McIntosh have served as executive officers for not less than the past five years.

Stephen C. Curley re-joined Olin on August 18, 2003 as Chief Tax Counsel. He was elected Vice President and Treasurer effective January 1, 2005. From 1997-2001, he served as Vice President and Treasurer of Primex Technologies, Inc., a manufacturer and provider of ordnance and aerospace products and services, which was spun off from Olin in 1996.

John E. Fischer re-joined Olin on January 2, 2004 as Vice President, Finance. On June 24, 2004, he was elected Vice President, Finance and Controller, and effective May 27, 2005, he was elected Vice President and Chief Financial Officer. From 1997-2001, he served as Vice President and Chief Financial Officer of Primex Technologies, Inc. During 2002 and 2003, Mr. Fischer did independent consulting for several companies including Olin.

G. Bruce Greer, Jr. joined Olin on May 2, 2005 as Vice President, Strategic Planning. Prior to joining Olin and since 1997, Mr. Greer was employed by Solutia, Inc., an applied chemicals company. From 2003 to April 2005, he served as President of Pharma Services, a Division of Solutia and Chairman of Flexsys, an international rubber chemicals company which was a joint venture partially owned by Solutia and Akzo Nobel. Prior to that, Mr. Greer served as a Vice President of Corporate Development, Technology, and Information Technology for Solutia.

Jeffrey J. Haferkamp was elected Vice President and President, Olin Brass effective January 1, 2005. Prior to that time and since October 2002, he served as President, Rolled Products. From April 2001 to September 2002, he served as Vice President, International for the Brass Division (currently part of the Metals Group). Prior to April 2001, he served as Vice President, Business Planning for the Brass Division.

Richard M. Hammett was elected Vice President and President, Winchester Division effective January 1, 2005. Prior to that time and since September 2002, he served as President, Winchester Division. From November 1998 until September 2002, he served as Vice President, Marketing and Sales for the Winchester Division.

Dennis R. McGough was elected Vice President, Human Resources effective January 1, 2005. Prior to that time and since 1999, he served as Corporate Vice President, Human Resources.

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George H. Pain re-joined Olin on April 15, 2002 as Vice President, General Counsel, and Secretary. Prior to the time, since 2001, he served as Vice President and General Counsel of General Dynamics Ordnance and Tactical Systems, Inc., an operating unit of General Dynamics Corporation, a manufacturer of mission-critical information systems and technologies; land and expeditionary combat systems, armaments and munitions; shipbuilding and marine systems; and business aviation. From 1997-2001, he served as Vice President, General Counsel and Secretary of Primex Technologies, Inc.

Todd A. Slater was elected Vice President and Controller, effective May 27, 2005. From April 2004 until May 2005, he served as Operations Controller. From January 2003 until April 2004, he served as Vice President and Financial Officer for Olin’s Metals Group. Prior to 2003, Mr. Slater served as Vice President, Chief Financial Officer and Secretary for Chase Industries Inc. (which was merged into Olin on September 27, 2002).

PART II

As of January 31, 2006, we had 6,029 record holders of our common stock.

Our common stock is traded on the New York Stock Exchange, Chicago Stock Exchange and Pacific Exchange, Inc.

The high and low sales prices of our common stock during each quarterly period in 2005 and 2004 are listed below. A dividend of $0.20 per common share was paid during each of the four quarters in 2005 and 2004.

Issuer Purchases of Equity Securities

Equity Compensation Plan Information

We incorporate the information concerning our equity compensation plans under the heading “Equity Compensation Plan Information” in our Proxy Statement relating to our 2006 Annual Meeting of Shareholders by reference in this report.

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Item 6. SELECTED FINANCIAL DATA

ELEVEN-YEAR SUMMARY