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- Óc EoTownship in Mekong Delta, Vietnam
- OctarOctar or Ouptaros was a Hunnic ruler. He ruled in dual kingship with his brother Rugila, p
- OdaenathusKing of Palmyra from 260 to 267
- Odrysian kingdomUnion of Thracian tribes and kingdoms (5th century BC to 3rd century BC)
- Old Assyrian periodSecond period of Assyrian history
- Old ChineseOldest attested stage of Chinese
- Old Chosŏn? – 108 BC state in East Asia
- Old Kingdom of EgyptPeriod in ancient Egyptian history (c. 2686–2181 BC)
- Old Pahang kingdomFormer kingdom in Pahang, Malaysia
- OlmecsEarliest known major Mesoamerican civilization
- OponeAncient proto-Somali trading center
- Optimates and popularesPolitical labels in the Roman Republic
- OrdonaOrdona is a small town and comune of the province of Foggia in the region of Apulia in sou
- OrolesOroles was a Dacian king during the first half of the 2nd century BC.
- Ostrogoths5th–6th-century Germanic ethnic group
- OualataCommune and town in Hodh Ech Chargui, Mauritania
- Overthrow of the Roman monarchySemi-legendary overthrow of the Roman monarchy and foundation of the republic
- Owari clanThe Owari clan is a Japanese clan. The clan were originally Kuni no miyatsuko but after th
- Owari Tokugawa familyThe Owari Tokugawa family (尾張徳川家, Owari Tokugawa-ke) is a branch of the Tokugawa clan, and