I was curious why O'Brien, an enlisted man, was the chief engineer for a critical space station. turns out the ds9 writers just wanted him to be enlisted, because on TNG he was an officer. Rodenberry wanted everyone on a spaceship to be an officer, like modern day astronauts. they just called him chief O'Brian on TNG becuase he was the transporter chief ,in the same way geordi was chief engineer. if you wanted you could make a silly retcon and say that his officers rank on the ship was a "field promotion" since all ship crews are officers, and when he got tranferred off to ds9 he went back to his original rank, but why bother? for that matter why bother making O'brian enlisted?