That's the new Starfleet, ladies and gentledudes. Cadets become Captains, and navigator ensigns become chief engineers.
Geordi became Chief Engineer in TNG without seeming to know anything about engineering (being sent down there or offering suggestions in Season 1). And not only that he was an expert from the get-go.
At least Ensign Chekov had problems there.
I don't disagree that Geordi's rank was an issue (he was a lieutenant and the assistant chiefs in season one had all been lt-commanders so we never saw one of those assistant chiefs again). However, there were references in season one to imply had engineering training, he was already a lt(jg) before being promoted, he was older, and there was never any express suggestion that he was positioned at the helm straight out of the Academy (in fact Memory Alpha confirms he was 12 years older than Chekov and the Enterprise was his second posting so if anything he'd spent a long time before getting promoted).
O'Brien's retconned rank was a blatant retcon but I'm glad they did - I get fed up only seeing senior officers and his history was all over the place (he'd been helmsman, engineer, tactical, and transporters) but he was on a non-Federation vessel with non-federation personnel and was vastly experienced. He would not have become a chief engineer on a starship but could get away with it on a small ship with few officers like the Defiant.