In "All Good Things" O'Brien's rank (past) is ensign. O'Brien is in a red uniform and wearing a single pip on his collar. Picard refers to him as Chief, but O'Brien doesn't correct him or seems surprised. Picard also puts him in charge of the plasma inducer modifications and then O'Brien gives an order to Ensign Fletcher, who responds aye sir. Nocoms are not refered to as sir.
Would this not confirm the theory that O'Brien's rank in the pilot episode was a field promotion? Due to having a skeleton crew?