An o-ring is a round, mechanical gasket designed to be seated in a groove and compressed during assembly between two or more parts, creating a seal at the interface. O-rings are one of the most common types of seals because they’re inexpensive, reliable and have simple mounting requirements. Since they are known as one of the most important elements of machine design, o-rings may be used in static applications, where little range of motion is required. O-rings may be used in dynamic applications, like rotating pump shafts and hydraulic cylinder positions, also.