PAO 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. Van Over and D. Sperber were working in or as PAO. It and the officers' names were listed on directory placards. (ENT: "Demons", directories)
PAO, like many of the other agencies and titles on the directories, was a reference to NASA and ISS terminology, in this case, the "Public Affairs Officer".