ACO was an agency or title held by a Starfleet officer at Starfleet Command's Fleet Operations Center, Sol Sector.
In January of 2155, Starfleet officers J. Nesterowicz and P. McWade were working in or as ACO. It and the officers' names were listed on directory placards. (ENT: "Demons", directories)
ACO, like many of the other agencies or titles on the directories, was a reference to NASA and ISS terminology, in this case, the "Assembly and Checkout Officer".