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- Obadiah (Khazar)Obadiah was the name of a Khazar ruler of the late eighth or early ninth century. He is de
- Occupation of Mongolia1919–1921 Republic of China occupation of Outer Mongolia
- OddiyanaKingdom in early medieval India
- Oghuz Yabgu State766–1055 Turkic state in Central Asia
- Oirat ConfederationConfederation of Oirat tribes of Western Mongolia
- Old DutchIndo-European language
- Old FrenchGallo-Romance dialect continuum
- Old Great Bulgaria632–668 nomadic empire in Eastern Europe
- Old JavaneseOldest attested phase of the Javanese language
- Old PrussiansHistorical Baltic tribal group
- Omnium in mentemOmnium in mentem (To everyone's attention) is the incipit of a motu proprio of 26 October
- On'yomiKanji reading based on the original Chinese
- Operation Coldstore1963 security operation in Singapore
- Operation Lalang1987 Malaysian police crackdown
- Operation Neptune (espionage)1964 Eastern Bloc disinformation operation involving fake Nazi-era documents
- Order of AssassinsNizari Isma'ili military order (1090–1256)
- Order of Saint John in the NetherlandsThe Order of Saint John in the Netherlands, also known as the Johanniter Orde, is a Dutch
- Order of SantiagoSpanish religious and military order founded in the 12th century
- Order of St. George (Habsburg-Lorraine)European dynastic order of chivalry
- Order of the Golden FleeceCatholic order of chivalry
- Order of the Three Golden FleecesThe Order of the Three Golden Fleeces (French: Ordre des Trois-Toisons d'Or) was an order
- Orio MastropieroOrio Mastropiero (died 13 June 1192), forename sometimes rendered as Aurio and surname as
- Ostrogothic Kingdom493–553 kingdom centered in Italy
- OstsiedlungEarly and High Middle Age German migration movement to the East
- Ottoman dynastyRoyal family of the Ottoman Empire
- Ottoman TurksTurkic ethnic group
- Ottoman–Mamluk War (1516–1517)Imperial Ottoman conquest of Egypt and the Levant
- Ottoman–Persian WarsSeries of wars through the 16th to 19th centuries
- Ottonian dynastySaxon dynasty of German monarchs (919–1024)
- Ottonian Renaissance10th-century cultural and literary movement
- Our Wars with the Burmese1917–1920 Thai history book by Prince Damrong Rajanubhab