O-Ring:  As long as the O-ring is on the battery case while it is screwed into the flashlight head it should stay clean and not need attention.  To insure the O-ring does not collect dirt never leave the battery case unattached to the flashlight head except for the installation of batteries, or pocket clip removal/reversal.  The Buna N O-ring should only be coated with silicone grease; as other greases may destroy the O-ring.  A spare O-ring coated with silicone grease along with a spare stainless steel roll pin for lanyard attachment is located in the holster in a plastic bag at the bottom of the battery pouch.  To remove the O-ring from the battery case insert 4 round toothpicks spaced equally apart under the O-ring and slide it off over the threaded end of the battery case.  Reverse this procedure to install a new O-ring.