Executive officer
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The Executive Officer (or "XO") is the second in command aboard a battlestar, reporting directly to the Commanding Officer.
Aboard battlestar Galactica this position is held by Saul Tigh. He is responsible for the day-to-day operation of the ship, and takes command in William Adama's absence.
The XO of battlestar Pegasus is Colonel Jack Fisk, who reports to Admiral Helena Cain.
Note
Executive Officer is a military term in this context, used in several branches of the armed forces worldwide (naval, marines, air force). Certain forces, such as the British Army use the term "2i/c" (signifying second in command) in preference to "XO."