| O Canada ... O'Higgins, Kevin Christopher |
| O'Jays, the ... Oates, Joyce Carol |
| Oates, Titus ... Obrenovic dynasty |
| Obscene Publications Act ... oceanic trough |
| oceanography ... Ode on a Grecian Urn |
| Ode on Indolence ... Oeneus |
| Oengus, Saint ... Ogot, Grace |
| OGPU ... Oise River |
| Oistrakh, David ... Okounkov, Andrei |
| okra ... Old High German |
| old maid ... Olgas |
| Olier, Jean-Jacques ... Olsen, Merlin |
| Olsen, Tillie ... Oman |
| Oman, flag of ... Onate, Juan de |
| Once and Future King, The ... Ontake, Mount |
| Ontario ... open-hearth process |
| open-market operation ... opposition, square of |
| oprichnina ... Orange |
| Orange ... Orchis |
| orchitis ... Orestes |
| Orestes ... oriole |
| Orioles, the ... Ormoc |
| ormolu ... Orsi, Paolo |
| Orsini Family ... oryx |
| Orzeszkowa, Eliza ... Osman Nuri Pasa |
| Osmanabad ... Osterdalen |
| Ostergotland ... Otis, Elisha Graves |
| Otis, Harrison Gray ... Ottonian art |
| ottrelite ... Ourense |
| Ourinhos ... overture |
| Overweg, Adolf ... Oxalidales |
| Oxalis ... oyster |
| Oyster Bay ... |
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