|Command Uniform||Aerospace Corps Duty Uniform||Marine Corps Duty Uniform|
A First Officer (also known as the Executive Officer, XO, or Number One) is the second-highest ranking officer aboard a starship or space station after the Commanding Officer. He or she takes command if the Commanding Officer is unable to.
In times of crisis, the XO may be designated as Commander Pro Tempore and is one of two officers capable of jointly removing the CO from command.
At the end of each mission, the XO and CO determine any promotions that have been warranted over that mission.