First Officer

Aboard a starship the First Officer or Executive Officer (or XO) was the second-in-command, just below the Captain.

Duties
In STARFLEET the position was usually held by an officer with the rank of commander or lieutenant commander.

The first officer served as a department head first and the executive officer second and only took command when the captain left the bridge, as such, there was no first officer's station. The primary role of the second-in-command was to carry out the decisions of the Captain and to safeguard the lives of the ship's crew, including the captain. He also had to manage the ship's resources, including systems allocation.

The first officer, if there wasn't the position of second officer, usually led away teams while the captain remained on the ship, but also reviewed and kept the ship's record of battle.