| O | Chemical symbol for Oxygen. |
| Occlusion | A term applied, in the case of metals, to the absorption or entrapment of gases. |
| Oil Hardening | Quench-hardening treatment involving cooling in oil. |
| Oil Hardening Steel | Used to describe tool or alloy steels where oil is used as the quenching medium in the hardening process. |
| Oil Quenching | Hardening of carbon steel through cooling in an oil bath at a desired rate. Oils are categorized as conventional, fast, martempering, or hot quenching. |
| Open Hearth Furnace | Developed in the middle of the last century, the open hearth or Siemens-Martins process, as it is known, accounted for a major proportion of UK steel production until the early 1970's. For economic and quality reasons it has been replaced by the Electric Arc Furnace and the Basic Oxygen Steelmaking process. There are no open hearth furnaces in use in Britain today but they are still in use in Russia and Eastern Europe. |
| Orange Peel Effect | An effect that arises on the surface of steel sheets when they are stretched beyond their elastic limit. |
| Ore | An ore is a material that contains a metal in such quantities that it can be mined and worked commercially to extract that metal. The metal is usually contained in chemical combination with some other element in addition to various impurities. |
| Os | Chemical symbol for Osmium. |
| Overaging | Aging under conditions of time and temperature greater than those required to obtain maximum change in a certain property, so that the property is altered in the direction of the initial value. See aging. |
| Overheating | Failure of tools and components in heat treatment can arise through overheating. This may be caused due to quenching from a temperature too high for the type of steel involved. Overheating is evidenced by cracking, grain-coarseness, erratic surface hardness and pitting. |
| Oxidation | A common form of chemical reaction which is the combining of oxygen with various elements and compounds. The corrosion of metals is a form of oxidation, rust on iron for example is iron oxide. |
| Oxy-Acetylene Welding | A process for joining two pieces of metal in which the required high temperature is obtained by the combustion of acetylene gas and oxygen. The gases are thoroughly mixed in the nozzle or tip of the welding torch to ensure perfect combustion. The weld may be formed directly between two adjoining surfaces, but usually metal from a welding rod is fused in between the surfaces of the joint. |
| Oxygen | Oxygen is one of the chief constituents of the atmosphere of which it forms approximately one fifth. It is odorless and invisible. Although oxygen itself does not burn it is extremely efficient in supporting combustion, nearly all other chemical elements combine with it under evolution of heat. It has many uses in industry and is essential to the BOS (Basic Oxygen Steelmaking Process). |
| Oxygen Probe | An atmosphere-monitoring device that electronically measures the difference between the partial pressure of oxygen in a furnace or furnace supply atmosphere and the external air. |
