(covers information from several alternate timelines)
Operations officers are responsible for overseeing internal systems control, communications and sensors, while also coordinating the scheduling of resources, hardware, and system usage for an entire starship, outpost, or space station. Operations officers manned the ops station.
The operations position was manned by a member of the command staff of a starship, regardless of seniority. On the starship USS Enterprise-D, Chief Operations Officer Lieutenant Commander Data (doubling as Second Officer) was the most senior member of the ship's crew, after the Captain and first officer, but 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, chief operations officers often performed the additional duties of a science officer, especially when a dedicated science officer was absent, as exemplified by both Data and Harry Kim. (TNG: "Conundrum"; VOY: "Caretaker", etc.) Operations officers also performed the duties of a communications officer. (citation needed • edit)