Below is a list of O-Rings manufacturers from King of Prussia, Pennsylvania. O-rings are round, ring-shaped molded rubber sealing devices, which are critical for creating seals between connecting pieces in machines belonging to nearly every industry. Named for their O shape, the function of o-rings are both extremely simple and extremely important. O-rings function similarly to the way gaskets do in connecting valves, tubes, vacuums and hydraulic flow applications, with an impermeable seal between two joints.