An O-ring is a torus, or doughnut shaped ring, though generally molded from an elastomer, O-rings can also be made from PTFE and other thermoplastic materials, as well as metals, both hollow and solid. O-rings are used primarily for sealing, but are also used for light duty, mechanical drive belts.

In designing an O-Ring Seal, it is best to determine the O-ring compound first, as the compound may have a large influence on overall design parameters. The application will determine the rubber compound with the primary factor being the fluid to be sealed. The elastomer however, must also resist extrusion when exposed to the maximum anticipated system pressure and be capable of maintaining good physical properties through the full temperature range expected. In dynamic applications, the selected material must also have the toughness and abrasion resistance so important in reciprocating and rotary seals.