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- Oda HidetakaOda Hidetaka (織田 秀孝, ? - July 14, 1555) was a Sengoku period warrior in 16th century Japan
- Oda NagamasuOda Nagamasu (織田 長益, 1547 – January 24, 1622) was a Japanese daimyō and a brother of Oda N
- Oda NobuhiroOda Nobuhiro (織田 信広, died October 13, 1574) was the eldest son of Oda Nobuhide. As an ille
- Oda NobukaneOda Nobukane (織田 信包, 1548 – August 22, 1614) was a Japanese samurai, the younger brother o
- Oda NobunagaJapanese samurai and warlord (1534–1582)
- Oda NobuyukiOda Nobuyuki (織田 信行, 1536 – 1558), also known as Oda Nobukatsu (織田 信勝), was the son of Oda
- Oda of HaldenslebenOda of Haldensleben (c. 955/60 – 1023) was Duchess of the Polans by marriage to Mieszko I
- Odebrecht CaseThe Odebrecht case is one of the largest corruption cases documented in recent Latin Ameri
- Offa of MerciaKing of Mercia from 757 to 796
- Office of Civilian DefenseOffice of Civilian Defense was a United States federal emergency war agency set up May 20,
- Ögedei KhanKhan of the Mongol Empire from 1229 to 1241
- Oguri KozukenosukeJapanese politician
- Ohio's 20th congressional districtThe 20th congressional district of Ohio was created after the 1840 census. It was eliminat
- Ōkubo ToshimichiJapanese statesman (1830–1878)
- Olaf HaraldssonKing of Norway from 1015 to 1028
- Olaf ScholzChancellor of Germany from 2021 to 2025
- Old KandaharAncient area of Kandahar in Afghanistan
- Old Style and New Style datesChanges in calendar conventions
- Oleg the WiseVarangian prince, founder of Kievan Rus'
- Olga of KievPrincess of Kiev, Orthodox saint (c. 890–925)
- Olive Risley SewardOlive Risley Seward (July 15, 1844 – November 27, 1908) was a writer and the adopted daugh
- Olive SoulouqueOlive Soulouque, Princess Imperial of Haiti (29 November 1842 – 23 July 1883) was a Haitia
- Oliver CromwellEnglish military and political leader (1599–1658)
- Olusegun ObasanjoNigerian head of state (1976–79; 1999–2007)
- OlympiasMother of Alexander the Great (c. 375–316 BC)
- Omar al-BashirLeader of Sudan from 1989 to 2019
- Omar Ali Saifuddien IIISultan of Brunei from 1950 to 1967
- Omar BongoPresident of Gabon from 1967 to 2009
- Omar Saidou TallWest African scholar and military leader
- Omar TorrijosMilitary leader of Panama from 1968 to 1981
- Onjo of Paekche1st King of Paekche (r. 18 BC – AD 28)
- Operation LaserOperation Laser is a domestic operation of the Canadian Armed Forces for contingency plann
- Opoku Ware IOpoku Ware I was the 2nd Asantehene of Oyoko heritage, who ruled the Ashanti Empire. Betwe
- Orda (organization)Type of organization
- Order of Brilliant JadeThe Order of Brilliant Jade is a civilian order of the Republic of China that can be worn
- Order of Propitious CloudsThe Order of Propitious Clouds (Chinese: 卿雲勳章) is a civilian order of the Republic of Chin
- Order of Saints Maurice and LazarusRoman Catholic dynastic order of knighthood
- Order of San CarlosThe Order of San Carlos is a state order granted by Colombia. The order was founded 16 Aug
- Order of Tahiti NuiThe Order of Tahiti Nui was established on 5 June 1996 by the Assembly of French Polynesia
- Order of the BathBritish order of chivalry established in 1725
- Order of the Crown of JohorThe Most Honourable Order of the Crown of Johor (Malay: Darjah Mahkota Johor Yang Amat Mul
- Order of the Crown of PahangThe Esteemed Order of the Crown of Pahang (Malay: Darjah Kebesaran Mahkota Pahang Yang Ama
- Order of the National Hero (Antigua and Barbuda)The Most Exalted Order of the National Hero is an Antiguan and Barbudan order of chivalry
- OrdonnanceIn French law, an ordonnance (French: , "order") is a statutory instrument issued by the C
- Ordoño II of LeónKing of León from 914 to 924
- Orléans CathedralOrléans Cathedral (French: Basilique Cathédrale Sainte-Croix d'Orléans) is a Roman Catholi
- Óscar AriasPresident of Costa Rica (1986–1990, 2006–2010)
- Óscar R. BenavidesPresident of Peru from 1933 to 1939
- Osei Kofi Tutu IMonarch of the Kingdom of Asante
- Osman IFounder of the Ottoman Empire
- Osterville, MassachusettsOsterville is one of seven villages within the town of Barnstable, Massachusetts, United S
- Oswald MosleyBritish fascist politician (1896–1980)
- Oswald of NorthumbriaKing of Northumbria from 634 to 641/42; Christian saint
- Oswaldo López ArellanoPresident of Honduras from 1963–71 and 1972–75
- Otto II, Holy Roman EmperorHoly Roman Emperor from 973 to 983
- Otto III, Holy Roman EmperorHoly Roman Emperor from 996 to 1002
- Otto IV, Holy Roman EmperorHoly Roman Emperor from 1209 to 1218
- Otto of BambergOtto of Bamberg (1060 or 1061 – 30 June 1139) was a German missionary and papal legate who
- Otto the GreatHoly Roman Emperor from 962 to 973
- Otto von BismarckChancellor of Germany from 1871 to 1890
- Ottokar II of BohemiaKing of Bohemia from 1253 to 1278
- Ottoman CaliphateIslamic domain under the Ottoman dynasty (1517–1924)
- Ottoman TurkishStandardized register of Turkish in the Ottoman Empire
- Ottoman Turkish alphabetArabic-based script for Ottoman Turkish
- Ottoman–Persian War (1743–1746)War between the Ottoman Empire and the Afsharid Iran (1743–1746)
- OusanasOusanas (fl. 320) was a King of Axum. Stuart Munro-Hay believes that it is "very likely" t
- Outsider in the White HouseOutsider in the White House is a 2015 political memoir co-authored by Huck Gutman and pres
- OvonramwenOba of Benin (1888 or 1889 until deposition in 1897)
- Owain ap Dyfnwal (fl. 934)Owain ap Dyfnwal (fl. 934) was an early tenth-century King of Strathclyde. He was probably
- Owain GlyndŵrWelsh rebel and pretender (died c. 1416)
- Owari ProvinceOwari Province (尾張国, Owari no Kuni) was a province of Japan in the area that today forms t
- Owen ArthurPrime Minister of Barbados from 1994 to 2008