O-rings are the most common hydraulic seal. They are used in O-ring face seal and O-ring Boss fittings, as well as many other components. They are seated into a recess or channel, sealing hydraulic system connections and preventing leaks around mating surfaces. Without them, it would be like a sprinkler system spraying hydraulic fluid or other chemicals. O-rings are essential to hydraulic systems, but they are not immune to failure.