I FEEL THE NEED...FOR SPEED.
Did the same mistake with the o-ring, and when I try to put the tank on the bike, fuel starts to flow from the fuel pump endless, Put the lines in but bike didn't start.Sorry for the late response. Yes, I'm talking about the internal o-ring with the plastic "cap" on it. Not sure if I assembled the pump correctly.
From the pic you show I think you assembled it right, the o-ring and the cup goes to the big part of the pump, and when you try to put together the 2 parts you'll notice a resistance meaning the o-ring seals in good.