The Officer of the Watch, also known as the Duty Officer or Officer of the Deck, is an officer placed in 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.
Individual decks on Starfleet vessels had officers of the watch. (DS9 novelization: Trials and Tribble-ations)
Examples[]
Lieutenant Donna "D.O." O'Neill served as Gamma shift commander on Enterprise. (ENT novels: What Price Honor?, Rosetta, The Good That Men Do)
In 2267, Lieutenant Nyota Uhura twice served as officer of the evening watch aboard the USS Enterprise. (TOS comics: "Thorpex", "Spectre of the Zond")
Lieutenant Russell Hexter served as a Beta shift Officer of the Watch aboard Columbia. (ST - Destiny novel: Gods of Night)
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")
Doctor Beverly Crusher occasionally served as Officer of the Watch for the night-shift on the Enterprise-D. (TNG episode: "Thine Own Self")
Commander Katerina Mueller was the dedicated night-shift commander aboard the USS Excalibur. (NF - The Captain's Table novel: Once Burned)