O-Ring or O-Ring is a toroidal-shaped gasket, usually made of rubber, whose function is to ensure fluid tightness, for example in hydraulic cylinders and pneumatic cylinders, as well as in water diving equipment. It is usually found in equipment to prevent the exchange of liquids or gases at the joints between removable parts.

The first patent of this board is dated May 12, 1896 (129 years old) as a Swedish patent. J. O. Lundberg"), the inventor of the O-ring, received the patent.[1]​ The US patent[2]​ was made in 1937 by 72-year-old Danish machinist, Niels Christensen").[3]​.

The O-rings are placed in grooves designed for this purpose in the closure elements, commonly shafts "Shaft (mechanical)") and covers.

There are various O-rings for different applications, such as depending on the material from which they are made. In that case, the working pressure and temperature to which it will be subjected are mainly observed.[4][5][6]​.

NASA's Space Shuttle Challenger disintegrated 73 s after launch due to the use of an O-ring that stopped sealing the SRB due to the use of inconvenient fluoroelastomers (FKM) at extremely cold temperatures before takeoff, thus losing its elasticity, and therefore, its ability to recover its shape after the expected movement of the gasket after the exit of the exhaust gases and due to its overcompression in the assembly.

O-rings need proper maintenance. Some recommendations for its use are the following: