O-rings are primarily used for sealing. Another application is the use as drive belts. As a seal, an O-ring is a means of preventing an unwanted escape of fluid or pressure. Types of seal applications include:

Static Seal

The mating gland parts are not subject to relative movement.

Dynamic Seal

1. Reciprocating relative motion along the shaft axis
2. Oscillating: movement around the shaft axis relative to the other member
3. Rotary: movement around the shaft axis in one direction only
4. Seat: mechanical distortion of the O-ring around a shaft in order to create the seal.

Pneumatic Seal

Similar to any of the ones previously described except it is used with a gas or a vapor rather than liquid.

Vacuum Seal

Seals a vacuum chamber or area.