The bit with starship hobby projects in "Second Sight" was S2, tho.
O'Brien was tactical officer on Rutledge. I thought: wtf?
DS9 "Paradise" goes to some length in explaining O'Brien's involvement in the first Setlik III incident. Combining this with what we learn from "The Wounded", the story seems to cater for what we get logically enough.
In 2347, Cardassians conduct "deniable" raids with small contingents of their militia rather than with actual military force. One hits the Setlik III colony, and the
Rutledge arrives too late to save much of the day. A set of landing parties beams down, probably including many top officers (as the skipper himself is personally involved there, his family being among the colonists), and verifiably including squads of troopers including the young O'Brien. In a firefight to protect the locals, O'Brien makes his first kill - accidentally, using a phaser set to kill by a civilian.
The militia then (earlier on?) corners some of the
Rutledge personnel. Their survival depends on the repairing of a field transporter, and trooper O'Brien manages this feat just in time to save at least some of the personnel.
It is in the aftermath of these events that O'Brien is said to have become the Tactical Officer. Not an improbable prospect at all: top officers beaming down, using rules of engagement calling for stun against a ruthless enemy, getting cornered, probably with heavy loss of life. When the heroic O'Brien returns to the ship, he may find the bridge thinly manned; his skipper may be among the people who owe their lives to him. And the location of it all is now considered "the Cardassian front", with the starship possibly isolated from crew rotation opportunities for some time.
O'Brien would at this point start wearing a gold uniform for the first time; he would probably have set his goals at becoming an engineer already, though, what with
a) him apparently not liking soldiering much if it involves killing
b) him already having a hobby background in engineering ("All Good Things..")
c) him having just saved lives through faking his way through an engineering task.
So he wouldn't rot at the Tactical position for long if he had any say about it.
The funny thing is, he'd apparently be back at Setlik III at some later timepoint, for an adventure he describes in "Empok Nor": he's fighting a surface battle there again, but this time against large formations of regular Cardassian military, and without mention of the
Rutledge. This may be the same incident where he and his
Rutledge buddy Boone fought as per "Tribunal" - an incident that apparently took place in 2362 rather than 2347, and probably didn't actually involve the starship because O'Brien is initially unsure whether Boone was present at that "Second Setlik" incident (something that would be natural if the two were simply buddies from the distant past when the shared a starship but not that close buddies at the time of the incident any more).
At "Second Setlik", O'Brien seems to be back to the old business of fighting as an infantryman. He even explicitly says in "Empok Nor" that he was a soldier back then. But he's been an engineer and transporter specialist since 2347, as per "Realm of Fear"... Perhaps he was both for a while (2347-62), and then gave up one profession in favor of the other (2362 and joining the E-D crew)? It's interesting enough that Meaney's character still serves as a security goon in an early TNG episode, "Lonely Among Us".
Timo Saloniemi
Though tbf we never saw Worf exhibit any eng practicalities, nor Geordi vice versa. But I doff my cap anyway 
