In the first Mass Effect game, After obtaining the Normady when you leave the Ship's VI says
XO Presley has the Deck.
and when you return it says
XO Presley stands relived.
Now i am assuming that since Shepard is the commanding officer Presley is second-in-command who takes command when Shepard is off the ship so when he returns Presley is relived of his command.
From my understanding, it's common that space-operating organizations in Sci-Fi are associated with the Navy (Time Space Administration Bureau's Dimensional Navy in Magical Girl Lyrical Nanoha, UNSC Marines in Halo and generally outer-space combat fights refereed to as Marines where Marines are normally under the jurisdiction of the Navy) but i don't think i've heard the term XO refereed to a second-in-command rather normally their rank is used
So what does XO mean? (like how CO = Commanding Officer)