OA – Order of the Arrow (Boy Scouts)
OBE – Order of the British Empire
OBLI – Ox and Bucks Light Infantry
OC – Order of Canada
OES – Order of the Eastern Star
OHC – Order of the Holy Cross
OM – Ordained Minister; Order of Merit
OMM – Order of Military Merit
OP – Order of Preachers (Roman Catholicism; Dominican)
OS – Old Style (referring to dates)
OSA – Order of St. Anne; Ordinis Sancti Augustini (of the order of St. Augustine)
OSB – Order of St. Benedict
OSIA – Order of the Sons of Italy in America
OSL – Order of St. Luke the Physician
OSM – Order of Servants of Mary
OSSB – Order of the Star Spangled Banner
ou – out of unity (Quaker)
OUAM – Order of United American Mechanics
