"In many militaries, an Executive Officer (XO) is the second-in-command, reporting to the Commanding Officer (CO). The XO is typically responsible for the management of day-to-day activities, freeing the commander to concentrate on planning the unit's next move. The XO also takes charge in the absence of the commander, and the position is frequently a stepping stone to a command position."
"Executive Officer (XO) In addition to RPG duties, the XO is responsible for assisting other members of the ship as needed. The CO may enlist the aid of his XO in fulfilling his ship management duties, but the CO will remain ultimately responsible for them. The XO's primary responsibility is to take over as acting CO (aCO) should the CO declare an LOA. An XO may declare the CO AWOL with the consent of the Fleet Commander and assume the position of acting CO until such time as the President appoints a new CO. A CO may appoint his own XO with the consent of the Fleet Commander, should the Fleet Commander choose to exercise such authority. No individual may hold more than one XO position simultaneously. The XO of Academy Training Ships does not count towards this limit. Acting XO does not count towards this limit."
The position of XO is one of support to the commanding officer as well as running your ship efficiently. In the chain of command all Department Heads report to you about the day to day running of their departments and the welfare of the crew. You then in turn report that to the CO.
An XO manages the ship effectively and efficiently making best use of the resources available. You manage your Department Heads, who in turn manage their own departments. You will interpret the wishes of your Commanding Officer and pass them down in the chain of command.
Just as a DH forms a bridge on the ship, the XO does too. You are the link between the Captain and the crew. Very few of the crew themselves will have direct contact with the Captain, since most situations and matters will be dealt with by the Department Heads or by you.
You are the first and foremost link in the chain of command, the Captain's right hand and immediate support, as well as the main reporting figure for the crew and department heads. You advise the Captain and interpret the CO's orders for your juniors.