This is the one thing that remains rather difficult to explain about Colm Meaney's constant rank identifier changes in the various shows. Dialogue basically agrees that he always was a noncom and never attended the Academy - so how did he become the Tactical Officer of the Rutledge?
Didn't Riker specifically call him "lieutenant in one episode?
The fact that no planning went into character until DS9 seems to have caused endless confusion among the staff writers. Until the sixth season of DS9, O'Brien wore the rank pips of a full lieutenant, and I have a fuzzy memory of Riker refering to him as such. He was the "transporter chief" -- a position, not a rank -- and so the characters usually referred to him as "chief." My own guess, fwiw, is that the writers soon confused O'Brien's position -- transporter chief -- with his rank -- chief petty officer-- and so O'Brien retroactively became an enlisted man.
R. Star wrote:
Trek's treatment of enlisted people is inconsistent as a whole really. DS9 sorta kinda tried to fix that but even they have O'brien routinely bossing officers around.
There's nothing inconsistent about that even in a modern military. Again, rank and position are two different things. In TNG, Data's position is second officer (third in command of the ship) even though Beverly outranks him. O'Brien is in an authority position, and those under his command -- whether enlisted men or junior officers -- have to listen to him.