Mind you, Tactical Officer need not be the guy pushing the phaser trigger buttons on a starship. Some of the Tactical Officers could well be specialists in planetside fighting, and O'Brien might have been one of those. I mean, that's how I'd make use of his skills if I were Captain Maxwell, freshly saved from the Setlik surface ambush by one of my Marines. O'Brien never demonstrated any skill in starship combat, really - the best he (his clone!) could do there was make a runabout come ahead in a chase in "Whispers".
Personally, I've always reconciled that "Tactical Officer" could be short form for "Petty Officer or Commissioned Officer assigned to Tactical (Operations)" however, as far as "not demonstrated any skill in starship combat" there is this gem from DS9: Rules of Engagement:
CH'POK: Chief, how many years have you been in Starfleet?
O'BRIEN: Twenty two.
CH'POK: And how many combat situations have you been in?
O'BRIEN: I couldn't even guess.
CH'POK: Try.
O'BRIEN: A hundred, hundred and fifty?
CH'POK: For the record, Chief O'Brien has been in two hundred and thirty five separate engagements and Starfleet has decorated him fifteen times. I would like to have him declared an expert in the area of starship combat.
We could also argue that (in-universe) the fact that he was assigned to Tactical during the tachyon blockade in TNG: Redemption part II over (presumably) dozens of specialists or officers from the Tactical/Security Division that could have taken over at the post.