No it's not. Benjamin Maxwell called O'brien his best tactical officer he ever had in the Wounded. In Rules of Engagement Worf described him as an outstanding officer. I guess since he's technically an NCO that could fly, being the last O does mean officer, but it gets even more strange when he routinely has officers(ensigns and lieutenants) working for him on DS9. O'brien does go out of his way several times to describe himself as an enlisted man, but calling him a Starfleet officer seems to be a catch all for everyone in Starfleet regardless if they hold officer rank or enlisted.