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- Oakwood mutiny2003 military incident in the Philippines
- ObeahAfro-Caribbean healing and spellcasting tradition
- Oblasts of the Soviet UnionThe oblasts of the Soviet Union were second-level administrative units of the Soviet Union
- Oblate Sisters of ProvidenceThe Oblate Sisters of Providence (OSP) is a Catholic women's religious institute founded b
- October 2025 No Kings protestsProtests against the Donald Trump administration
- October Manifesto1905 document by Tsar Nicholas II on restructuring Russian civil rights and government
- OeagrusMythical character, father of Orpheus
- Off the derechTerm for someone who has left an Orthodox Jewish community
- Official historyAn official history is a work of history which is sponsored, authorised or endorsed by its
- OiumOium was a name for Scythia, or a fertile part of it, roughly in modern Ukraine, where the
- Old English literatureLiterature of Anglo-Saxon England
- Old NorseNorth Germanic language
- Old Norse philosophyPhilosophy of the Norse peoples
- Old Testament theologyOld Testament theology is the branch of Biblical theology that seeks theological insight w
- Old YishuvOttoman-era Jewish community in Palestine
- Oliver Wendell Holmes Sr.American poet, essayist, physician (1809–1894)
- OlteniaHistorical region of Romania
- OmnerAccording to the Book of Mormon, Omner () was a Nephite missionary and one of the sons of
- On the Restoration of Independence of the Republic of Latvia1990 Latvian legal document restoring the country's independence
- One-party period of the Republic of TurkeyPeriod of Turkey from 1923 to 1945
- Ōnin War15th-century civil war in Japan
- OntotheologyBranch of theology
- Opatów GhettoThe Opatów Ghetto was a World War II ghetto set up by Nazi Germany for the purpose of pers
- Open OrthodoxyJewish religious movement
- Operation Harvest Festival1943 massacre of Jews during the Holocaust
- Operation LegacyBritish efforts to destroy records of wrongdoing
- Operation Menu1969–1970 US covert military operation in Cambodia
- Operation Nickel Grass1973 US airlift to Israel in the Yom Kippur War
- Operation SandwedgeOperation Sandwedge was a proposed clandestine intelligence-gathering operation against th
- Operation SpannerOperation Spanner was a police investigation into same-sex male sadomasochism across the U
- Opposition to Vladimir Putin in RussiaOpposition to the government of President Vladimir Putin in Russia, commonly referred to a
- Oracle bone scriptAncient form of written Chinese
- Orange River ColonyBritish colony from 1902 to 1910
- Orders, decorations, and medals of SudanList of the Orders, decorations and medals of Sudan.
- Organization of the Jews in BulgariaThe Organization of the Jews in Bulgaria (OJB) coordinates the different Jewish communitie
- OrientalismImitation or depiction of Eastern cultures
- OrigamiUSAOrigamiUSA (sometimes abbreviated as "OUSA") is the largest origami organization in the Un
- Origin of the RomaniansEthnogenesis of Romanians
- Origins of societyTheories of social origins
- Origins of the American Civil WarThe origins of the American Civil War were rooted in the desire of the Southern states to
- Orin Grant LibbyAmerican historian (1864–1952)
- Oromo peopleCushitic ethnic group
- OrphismSet of ancient Greek and Hellenistic religious beliefs
- OsemwendeOsemwende (c. 1780–1848), originally known as Prince Eredia-uwa, was the thirty-third Oba
- Otto II, Duke of SwabiaOtto II (c. 995 – 7 September 1047), a member of the Ezzonid dynasty, was Count Palatine o
- OudoceusSaint Oudoceus (Latin) or Euddogwy (Welsh) (c.536–c.615 or 625) is generally known as the
- Outright ScotlandOutright Scotland is an LGBT rights organisation based in Edinburgh, Scotland. Founded as
- Overseas ChineseEthnic Chinese residing outside of China
- Oxford Classical DictionaryThe Oxford Classical Dictionary (OCD) is generally considered "the best one-volume diction