O-ring 18x2 of oil plug - Simson NBR (Perbunan®, Europrene N®, Krynac®) - Nitrile rubber (oil resistant rubber, for standardized sealing technique) An O-ring is a rubber seal that is designed to prevent unwanted gas or liquid leakage or leakage. The sealing ability is not only dependent on the O-ring, but also on the installation space.