OSO was an agency or title held by a Starfleet officer at Starfleet Command's Fleet Operations Center, Sol Sector.
In January of 2155, Starfleet officer M. O'Halloran was working in or as OSO. It and the officer's name was listed on directory placards. (ENT: "Demons", directories)
OSO, like many of the other agencies or titles on the directories, was a reference to NASA and ISS terminology, in this case, the "Operations Support Officer".