Just to add to the mix, in the Royal Navy the second in command of a ship of frigate or destroyer size or above (and indeed nuclear submarines) has been known as the Executive Officer for at least the last 50 years. In larger ships such offiicers of Commander's rank will also be known by officers as "the Commander" or, more familiarly "the Bloke", whilst in smaller ship,s if a Lieutenant Commander or Lieutenant, he will be known as the First Lieutenant or, more familiarly, as "Number One" by officers, or as "Jimmy the One", or just plain "the Jimmy".
Hope this helps......
Jack