O-ring is a small, flexible ring that is used for ear plugs and some other body jewelry types to prevent them from falling off. Typically made of silicone, rubber or another soft material, O-rings are easily stretchable and can be positioned on jewelry without a problem. Their goal is to sit right at the piercing opening and to prevent the jewelry piece from slipping out. O-rings are mainly used for larger gauge jewelry pieces made for stretched piercings (most commonly, stretched earlobes).
Since many body jewelry pieces have some sort of a closing mechanism, O-rings are used only on those jewelry types that can’t be closed but can slip out. The most common type of such jewelry are smooth ear plugs. These plugs don’t have flares so they can easily slip out of the piercing hole. An O-ring is placed on the each side of the ear to prevent the ear plug from slipping out. O-rings are inexpensive and very handy so it’s useful to buy them in a pack and to use them whenever you need one. Keep in mind that these rings are easily lost so it’s always best to have some replacement O-rings ready if you lose the original ones.