Executive officer

The executive officer (or XO) is a billet position in the armed forces. They are not considered command ranks, but staff positions.

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 senior officer below the battalion commander. This post is usually held by a Captain or Major, though it may be held by a First Lieutenant.

Known executive officers

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