O'Brien was basically the "chief engineer in all but name" for the Defiant, one of the most accomplished war vessels, as station "chief engineer in all but name" he had to keep a gigantic space station based on alien technology (where there's few tech manuals on hand I would imagine) on the go as well as to help repair ships docked at the station. LaForge is very good n' all, but he really has a very easy time of it compared to O'Brien.
Your raise a good point about the Defiant, when it was first brought onto the show it would shake itself apart if pushed to max, plus despite all its firepower still gets its ass handed to it it their first encounter with the Jem'hadar, after that it becomes the most accomplished war vessel the federation had, Laforge started with working state of the art tech and maintained it pretty well, but the Defiant basically is re-built under O'Brien's supervision, fixes the inherant issues that the defiant had in the testing phases, upgrades the defiant to include ablative hull.
But not only that, he equips the station with weapons to defend itself, from scratch since the Cardassians removed all existing weapons from the station before leaving.