| O Canada ... O'Neill, James |
| O'Neill, John ... Oberhausen |
| Oberlin ... Ocean City |
| Ocean City ... Oda Nobunaga |
| Odaenathus, Septimius ... Offa |
| Offa Of Angel ... Ohira Masayoshi |
| Ohlin, Bertil ... Okene |
| Okha ... Olcott, Henry Steel |
| old age ... oleograph |
| Oleron Island ... Olson, Elder |
| Olsztyn ... Ona |
| Ona, Pedro de ... ontology |
| Onverwacht series ... Ophioglossaceae |
| Ophir ... opus anglicanum |
| Opus Dei ... orb |
| orb weaver ... Oregon Caves National Monument |
| Oregon City ... orientation |
| orienteering ... Ormandy, Eugene |
| Ormoc ... Orsted, Hans Christian |
| Orta, Lake ... Osborn, Henry Fairfield |
| Osborne, Dorothy, Lady Temple ... Ossian |
| Ossianic ballads ... Ostrovsky, Aleksandr Nikolayevich |
| Ostrow Wielkopolski ... Ottawa |
| Ottawa Agreements ... Ouidah |
| Ouija board ... overtone |
| overture ... oxbow lake |
| Oxenstierna, Axel, Greve ... Ozal, Turgut |
| Ozamis ... |
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