The United Services Organization, better known as the USO, was a volunteer organization formed in 1941 that focused on entertaining and comforting American troops overseas during and after World War II.
In 2375, during the Dominion War, after recording a few songs for Julian Bashir on an isolinear rod, Vic Fontaine offered to put together a regular USO show for the war-weary Starfleet troops who were about to make a supply run to AR-558.
He elaborated by stating that he wished he could go along so that he could make a few personal appearances, sing a few songs and tell a few jokes, to ease tensions. Unfortunately this was impossible as there were no holosuites on AR-558. (DS9: "The Siege of AR-558")