Washer for 3/4" piping with O-ring groove

I was hooking a garden hose faucet to a rain water collection barrel and needed a washer to help hold the O-ring in place and make it water tight. The faucet was 3/4" female input, and I got a threaded male adapter to go through the barrel, but the lip on it was so small, the o-ring kept slipping around it and leaking.

I screwed this piece down the male adapter, then slid the O-ring down into the groove, then screwed the piece through the side of the hole I made in the 55 gallon barrel. Then