Master Chief Petty Officer in the US Navy (which is where the Trek ranks come from) is E-9, which is as high a pay-grade as it gets for non-coms. While technically an Ensign outranks him, practically that's not how it works. Probably quite similar with Starfleet. As far as rank goes, O'Brien doesn't really have anywhere to go. His promotions will come from changes in posting, like to the Academy. If he were to get a commission, he would actually lose
prestige and position if he weren't immediately bumped up to Captain.
EDIT: Although, it looks like O'Brien was a Senior Chief Petty Officer, not Master Chief. So he still has a promotion ahead of him in about ten years or so.