These orings are used on the top of the CO2 tank that threads in to your gun.

One of the most common replacement items.  This oring screws in over threads every time the tank is installed or removed.  Easy to cut or nick.  Extras are always handy to have.  They are easy to replace and a must have to attach your air source.

It's a good idea to put a drop of oil on this oring everytime you screw your tank in.  The oring will last a lot longer.