This is what the US Owner's manual says:
Note it says it goes in front of the piston, not on the piston. It says it goes on the magazine tube in front of the piston, and that is exactly where you should put it.
To put this another way, here is the stack-up on the magazine tube -- first the big spring, then the piston, then the o-ring. Finally the cylinder of the barrel is placed over the tube, which captures the o-ring between the piston and the cylinder welded to the barrel.
The O-ring will have no trouble sliding up onto the piston by itself -- just get it in-between the piston and the cylinder.
If you lose the O-Ring, I am selling them here.
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