I'm sure that if they realised that they were going to use O'Brien as much as they did during TNG's run they would have given his character some more fore-thought.
Definitely so. But that probably would not have entailed making him a commissioned officer, as the whole point of the character was that he was the lowly everyman.
we found out he was a tactical officer on a Light Cruiser.
We never learned what sort of a ship the Rutledge
was, on screen. Off screen, she's a frigate of New Orleans
class, about the size of Kirk's old ship, but none of that leaks to the onscreen universe.
While "The Wounded" says O'Brien became the Tactical Officer of that ship, the episode also inherently establishes that he didn't stay in that position for long. "Paradise" shows the exact circumstances by which the Setlik III event changed his life (he saved a landing party by learning to repair a field transporter); "Realm of Fear" establishes the exact time (2347); and "Tribunal" and "Empok Nor" show that decades after this event (2362), O'Brien was still a humble foot soldier, not yet an engineer or even a transporter operator (but still associated with the Rutledge
). The TO thing was probably but a stint in his career.
Interestingly, "Paradise" has O'Brien start wearing gold as the result of the Setlik III thing. Is that an appropriate color for a Tactical Officer? We have seen it worn by a combined Tactical and
Security Officer in TNG and VOY, but such a combination may have been a very rare thing in Starfleet. Kirk's Tactical Officers in "Arena" did not wear the "old gold" or red - they wore the "old red" and blue.
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.
We see O'Brien wear a range of confusing rank symbols, but verbally we know a couple of definite things.
1) He's a Chief Petty Officer of some sort in "Family" - Sergey Rodzhenko should know.
2) He got a promotion when transferring to DS9, as per "A Man Alone" - but that may have been a promotion in position, rather than rank, as he stands to lose it if he refuses to continue serving as DS9 Chief of Ops.
3) He's a Senior Chief, Specialist in "Shadowplay".
4) He's a Chief Petty Officer in "Hippocratic Oath".
Essentially, all of the above would be consistent with the rank of Senior CPO (Specialist), some being more general expressions of that exact rank than others. And the collar rank marking he eventually gets to wear supports this rank as well: it features three chevrons (meaning he has gathered all the PO ranks and is now going through the CPO ones) and two dots or stars (meaning he's into his second CPO rank, which is Senior).
alternately speculate that his single dark pip before "Hippocratic Oath" was an "abbreviation" of a collar marking that has three chevrons and one dot or star, meaning he was a Senior CPO until that point and became a Master CPO at acquiring the two-dot collar plate. This, too, would be wholly consistent with the dialogue.
And in current USN practice, two stars actually denotes Master CPO, because there's a so-called "rocker" added to the chevrons to indicate "mere" or "plain" CPO, thus making the first star actually the second CPO rank, that of Senior CPO... It's just that the Starfleet plate has no rocker! And having the plate can't be what replaces having a rocker, because O'Brien would need a rocker or its plate equivalent to go with Senior CPO rank already.
In the end, dialogue presents no problems with having O'Brien finish DS9 as either SCPO or MCPO. Visuals on the costuming of Colm Meaney are inconsistent with any theory, but a little bit of squinting can accommodate a wide range of theories.