Besides leading a facility when the CO is unable to, the XO often assists the CO and serves as liaison with the crew. S/he oversees a ship or base's day-to-day operations and advises the CO on important matters.
Sometimes called the Executive Officer (XO) the First Officer's main responsibility is to ensure the smooth operation of the ship and her crew. In this regard, he is usually authorized (unless specifically limited by the CO in any respect) to make decisions without the CO's prior approval. He is considered, for all practical purposes, a "captain in training," and, in the absence of the CO, is the ship's commanding officer. He must be a jack-of-all-trades, able to answer almost any question or offer a solution to any problem. All department heads report directly to the First Officer. The FO is usually the leader of most away teams (especially potentially hazardous missions) and may choose members of those teams as long as his choices are not detrimental to the mission or the safety of the ship. For all practical purposes, the FO's staff is also comprised of the bridge crew and is also always on call. On larger starships, the FO usually has his own office. On smaller ships, he may be required to share his office with the CO or the Second Officer, or use a conference/meeting room for this purpose. The FO is also responsible for maintaining discipline and determining punishment for members of the crew found guilty of breaking minor regulations.