I think by the time they finally figured out what O'Brien's rank was, he was probably a senior chief petty officer (at least his insignia was closest to one in the U.S. Navy anyway). But if I understand it correctly, as a noncom he could still be a department head if so appointed by a senior commissioned officer.
It may also have been that he was the highest ranking person in the department if everyone else was an enlisted below the rank of chief, IMO. I could also see something similar like that for Harry Kim staying an ensign for 7 years if he was the only officer in the Voyager's operations department to begin with and other positions like chief engineer or tactical officer had higher billets...
It may also have been that he was the highest ranking person in the department if everyone else was an enlisted below the rank of chief, IMO. I could also see something similar like that for Harry Kim staying an ensign for 7 years if he was the only officer in the Voyager's operations department to begin with and other positions like chief engineer or tactical officer had higher billets...