This o-ring is found on the brass check valve (3343) of the Hatsan Velox PCP Pistol.
This valve makes sure that air can enter through the quick connect fill port and closes when you depressurize the fill source.

If your Velox is leaking through this fill connection, this is likely the cause!

Tip: to extend this seal's life, lubricate the ring with a little silicone grease.

Doubt if this is the part you need? Check the schematics!