I'm jealous..I'm still waiting on mine. From what I've read on here about them, the function of that o ring is mainly to squeegie the mouthpiece when it is removed. A few people have had theirs come out. I think one managed to get it to snap back into place, but most left it out with no effect on the performance of the carto. The vendor has suggested to just be aware that that piece is there and be a bit more cautious when removing or replacing the tip and/or when cleaning the unit.
edited to add: How are these working out for you despite the o ring issue. I'm stalking the mailman waiting for mine to get here to try them out. I've been a die hard ce2 user for awhile and if they can perform like my ce2s without the hassles, I'll be in
vape heaven!