I do have to wonder why Nog is OBrien's assistant?
I didn't get that he was. I thought they were basically co-Chief Engineers.
Because of course strictly speaking Nog is very much O'Brien's senior in terms of rank (commissioned vs non-commissoned) even if he is very much junior in terms of experience. If anything O'Brien should be Nog's assistant, and it's only because of O'Brien's length of service and respect from the officers that he's as in as senior a position as he is.