Executive Officer of an army unit. Normally the second in command of a unit as small as a company and as large as a brigade. The XO is responsible for the staff, logistics, maintenance and basically making sure everything runs smooth for the commander. At the company level its usually a 1LT, battalion level a MAJ and brigade a LTC. At higher echelon units, the XO is called the deputy commander or assistant division commander.
"The XO wants all the vehicles squared away before the long weekend"
"You better get your 4187 signed by the commander before he goes on leave otherwise you'll have to play 20 questions with the XO when he is the acting commander"