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- OdawaIndigenous people of North America
- Oirat languageCentral Mongolic language
- Olav Engelbrektsson28th Archbishop of Norway
- Old Swiss Confederacy1291–1798 confederation of Swiss cantons
- Old UyghurExtinct Siberian Turkic language of Central and East Asia
- Oliviero CarafaOliviero Carafa (10 March 1430 – 20 January 1511), in Latin Oliverius Carafa, was an Itali
- Oneida peopleIndigenous people of the United States
- Onondaga peopleEthnic group
- OprichninaState policy of Ivan IV (1565–1572)
- Oratory of Saint Philip NeriRoman Catholic society of apostolic life
- Order of Augustinian RecollectsMendicant Catholic religious order of friars and nuns
- Order of Friars Minor CapuchinReligious order of Franciscan friars
- Oromo expansion16–17th century northerly expansions of Oromo people
- Orsini affair1858 assassination attempt on Napoleon III
- Ottoman conquest of Otranto1480–1481 invasion of southern Italy
- Ottoman TripolitaniaSemi-autonomous state affiliated with the Ottoman Empire (1551–1912)
- Ottoman wars in EuropeSeries of military conflicts between the Ottoman Empire and various European states
- Ottoman–Habsburg wars1526–1791 series of wars in Europe
- Oudh StatePolity in the Awadh region of North India (1732–1856)
- Our Lady of Angels Church, KaraikalOur Lady of Angels Church (also called Karaikal Church) is the oldest church in Karaikal i
- OxenstiernaThe Oxenstierna family ( UUK-sən-shair-nə, OK-, Swedish: ) is a Swedish noble family, orig
- Oyo EmpireFormer empire in present day Nigeria and Benin Republic