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- Obizzo, Giovanni Maria (artist)
- Oceanus (Roman god)
- Oda Kaisen (artist)
- Odin (Norse god)
- Odundo, Magdalene (artist)
- Odysseus (legendary hero)
- Offa (king of Mercia)
- Ogata Kōrin (artist)
- ōhara Donshū (artist)
- Oignies, Hugo d' (goldsmith)
- Okada Kanrin (artist)
- Okamoto Toyohiko (artist)
- Okumura Masanobu (artist)
- Olaf III Haraldsson, 'the Peaceful' (king of Norway)
- Olbrich, Joseph Maria (designer)
- Oliver, Isaac (artist)
- Olof Skötkonung (king of Sweden)
- Olokun (Edo god)
- Omai ('a native of Ulaietea')
- Onchi Kōshirō (artist)
- Ono Tadashige (artist)
- Onoe Baikō VI (Kabuki actor)
- Onogi Gaku (artist)
- Oppenheim, Max von (archaeologist)
- Orophernes (king of Cappadocia)
- Orpheus (Greek legendary hero)
- Osiris (Egyptian god)
- Osorkon I (king of Egypt)
- Osorkon II (king of Egypt)
- Osorkon III (king of Egypt)
- O'Soup, Louis (Plains Ojibwa or Saulteaux leader)
- Ostade, Adriaen van (artist)
- Osulf (moneyer)
- Osun (Edo god)
- Otto I (Holy Roman emperor)
- Otto III (king of Germany)
- Owen, Robert (socialist reformer and philanthropist)
- Ozymandias ('king of kings')