O-Rings are the most important and most delicate part of every paintball. They are made of soft rubber, and are required to be in perfect condition for your marker to operate properly. If they get old, or dry out, your gun will not work. If they get too dirty they can not create a seal and your gun will leak. It is a very smart idea to have 1-3 of each o-ring you need on hand. Otherwise, you could have a wasted trip to the paintball field and or get less than optimal performance from your marker. Both scenarios take you out of the game.