O-Rings come in three basic sizes, Slim line, Regular and Heavy Duty (BB).  The Pentek basic O-rings fit most filter canisters.  Omni Filter is an exception and has their own line of O-Rings that are used in their canisters.  Proprietary canisters usually require special O-rings and can only be purchased through the manufacturer.  In some cases proprietary systems use standard canisters and Pentek O-Rings will work.  Call us if you have questions, 888-949-8883.