It's not a matter of O'Brien not having the experience or the training; he's probably more highly trained than either LaForge or Scotty, considering his assignment history. It's a matter of explaining why O'Brien hasn't been mustanged and promoted to at least Lieutenant already, to match his training and expertise.
And later on, he's basically performing administrative tasks, coordinating repairs to ships rather than actually doing the repairs himself.
Which, while making O'Brien appear mightier than either his training or his rank might warrant, is IMHO much better than him tuning the engines of the Prometheus
in "Second Sight", as if the giant starship with her supposed hundreds of crew did not have sufficiently clever engineers aboard!
Dax : "I have to meet with Chief O'Brien. We're boosting the maximum speed of Seyetik's ship to warp nine point five. If his experiment fails and that sun goes supernova, we're going to need to get out of there fast."
Or perhaps Dax was saying she needed to meet with O'Brien for the key to the spares locker, so that the starship's crew could do the modifications and Dax could watch over their shoulders, and O'Brien could stay on the station to scrub conduits or whatever...