Not that I have any problem with Scotty being next in line, when I built my master crew roster, I have:
Captain (O6) == Commander of the ship == runs the mission
Exec Officer / First Officer (senior O5) == Second in command == runs the ship
Operations Officer (junior O5 / senior O4) == Third in command == runs the front part of the ship
The three above often are not scheduled to sit in Center Seat for entire shifts at a time, but may fill in occasionally. During combat, one will be stationed in Aux Con instead of being on the main bridge.
Chief Navigator (senior O4) == senior Bridge Watch officer, de facto fourth in command
three or four Navigator (O4 or senior O3) == Bridge Watch Officers working a rotating duty schedule
These are the guys/gals who typically sit in the Center Seat during routine (i.e., BORING) patrol missions when nothing is happening, leaving the top three officers time to do all the command stuff (e.g., paperwork).
Chief Engineer (O5 or senior O4) == runs the back of the ship
Chief Weapons Officer (O4 or senior O3) == may fall under Ops Officer or be separate department under Exec
These two may or may not be in the line of succession, depending on their training and experience and the whims of the ship's Captain. They rarely have time to pull shifts in the Center Seat, depending on the experience level of their respective deputies.
Of course, we can't forget the Master Chief aka Chief of the Boat. He owns the ship and allows the officers to use it.
