O'brien was definitely a non-commissioned officer yet he issues orders to Ensigns! What gives?
One might say the station was originally intended to be run with a really small Starfleet crew, with no Starfleet officers serving in the Ops department running the station. That soon changed when the new mission of protecting the wormhole required the services of a few engineering specialists of commissioned rank, but there wasn't a pressing need to replace O'Brien, or to mustang him to Ensign or higher.
Alternately, Ops is still a completely enlisted-run operation, and O'Brien just commandeered one of the Ensigns from the Security department to do his bidding (we've seen plenty of examples of such cross-training of yellowshirts elsewhere in Trek). But it seems rather logical to have a few of O'Brien's techs hold officer rank due to their academic credentials or whatnot, yet still bow to the Chief for the benefit of the overall organization.
Timo Saloniemi