The Fleet Operations Center was a Starfleet Headquarters facility. This center was located in Marin County, a region north of San Francisco, on Earth. The personnel who worked at this facility oversaw Starfleet Operations. The center was first used by the United Earth Starfleet Command, then, after 2161, by the Federation Starfleet Command. (TNG: "Conspiracy"; Star Trek VI: The Undiscovered Country; ENT: "The Expanse", "Demons", "Terra Prime")
In January 2155, the Fleet Operations Center was the location of the various conferences held by the delegates of Earth and various other planets to discuss a possible Coalition of Planets. Various directories were placed throughout the area, listing the building's personnel and their offices. (ENT: "Demons", "Terra Prime")
In 2293, the Fleet Operations Center was involved in the planning for "Operation Retrieve". (Star Trek VI: The Undiscovered Country, production art)
In 2364, while investigating an alleged conspiracy within Starfleet, Captain Jean-Luc Picard visited the Fleet Operations Center. (TNG: "Conspiracy", set artwork)
The Fleet Operations Center was first mentioned on a directory seen in the Star Trek: The Next Generation episode "Conspiracy". The Fleet Operations Center then became a major location for Star Trek: Enterprise which set several scenes there and clarified this was a separate facility from Starfleet Headquarters. Although not mentioned by name, the large control room seen in Star Trek IV: The Voyage Home was also most likely set within the Fleet Operations Center.