(covers information from several alternate timelines)
The operations officer, also referred to as the operations management officer, was a Starfleet position within the operations division which was common in the 24th century. Operations officers manned the ops station on a bridge.
On the starship USS Enterprise-D, Lieutenant Commander Data served as the operations management officer and doubled as Second Officer, the most senior member of the ship's crew after the Captain and First Officer. On the starship USS Voyager, Operations Officer Ensign Harry Kim was the most junior bridge officer.
As analytical operations relied upon knowledge of science and engineering disciplines, operations officers often performed the additional duties of a science officer, especially when a dedicated science officer was absent. (TNG: "Conundrum"; VOY: "Caretaker", etc.)