Executive officer

The executive officer (or XO) is a billet position in the armed forces. They are considered command positions, and is always the officer that is junior to the commanding officer.

UNSC Navy
In the UNSC Navy, the executive officer (or First officer) is the senior most crew member below the commanding officer of the ship. Usually, this post will be held by a Commander or Lieutenant Commander, but occasionally will be held by a Lieutenant.

UNSC Army and Marine Corps
In the UNSC Army and Marine Corps, the executive officer is the officer that is second-in-command to the commanding officer. Executive officer billets exist from the company level up, in both the Army and Marine Corps.

Note that platoons do not have XOs; the second-in-command in a platoon is a platoon sergeant.

Known executive officers

 * First Lieutenant Melissa McKay, ODST division on the
 * Commander Richard Lash,
 * Lieutenant Commander Julian Waters, (acting)
 * Armstrong,