Which skimmer is it and which o-ring?
If it's an RC, I'm there for you :x Every time I take the pump off it blows apart like a defective robot. I usually lay it on it's side and put the o-ring on and then the pump assembly. If you snug the pump up to the unions as you screw on the retainer ring it stays in place too.
If it's the one on the skimmer cup, which if it's rolling off it probably is, I pulled it out away from the plexiglass ring a section a little at a time and moved it down so it wouldn't roll because if I tried to roll it on it would unroll and shoot off. As you pull it back roll in a bit so when you get to the base it stays on. You could try rolling it half way and then slide it over so it rolls the way it wants once it's on but that is tricky.
