The Officer of the Watch, also known as the Duty Officer or Officer of the Deck, is an officer placed temporary command of a vessel while the commanding officer is not present on the bridge. During normal operations this duty would fall to the Executive Officer or Second Officer, especially during Alpha and Beta shifts. It is more likely to find an officer from outside regular senior bridge staff, commanding the Gamma shift.
Lieutenant Commander Data frequently acted as Officer of the Watch, on the USS Enterprise-D, during the night shift, since he did not require sleep. (TNG episodes: "Data's Day", "Lessons", "Rightful Heir").