Also known as a toric joint — in reference to its torus shape — an O-ring is a gasket with a circular-looped design that's used to seal two working surfaces. The valve stems in automotive engines, for example, often use O-rings to prevent oil from entering the combustion chamber.The main purpose of O-rings is to prevent fluid or air from escaping as it acts as a sealing device. When the ring is squeezed between two surfaces, it takes up the clearance and then prevents any fluid or air from being released.

The rod seal is the most critical seal on any type of fluid power equipment preventing leakage of fluid from within the cylinder to the outside.Common pneumatic seals include piston seals, V-rings, wipers, symmetrical seals, rotary shaft seals, wear rings, static seals, buffer rings, back-up rings, u-cups, gaskets, and even an O-ring.