The concept of Warrant Officers came from the Royal Navy where regular officers were from the upper class and ordinary seamen from the lower class. Eventually it dawned on someone that a lot of capable men were being buried in the enlisted ranks. It wouldn't do to have lower class types commanding ships, but the warrant concept at least gave the exceptionally talented enlisted types a chance at better privilege and pay. Making O'Brien a warrant officer would have easily solved the riddle of how he could have served as "Tactical Officer" on board another ship without being an Academy graduate. Of course, in the U.S. Navy the "Diving Officer" on submarines as I understand it is at least some of the time the Chief Enlisted man, known as "Chief of the Boat" or COB.