| o association ... O'Donnell, Hugh |
| O'Donnell, Hugh Roe ... O'Neill, Rose Cecil |
| O'Neill, Shane ... Oakham |
| Oakland ... Obando, Jose Maria |
| Obasanjo, Olusegun ... object-oriented database |
| object-oriented language ... Obote, Milton |
| Obradovic, Dositej ... Occidental, Cordillera |
| Occidental, Cordillera ... Oceanic literature |
| oceanic lithosphere ... Ocotal |
| Ocotea ... Odaira Namihei |
| Odantapuri ... odontophore |
| odontostome ... Oeuvre, Theatre de l' |
| Ofen, Treaty of ... Ogata Korin |
| Ogata, Ken ... Oh Sadaharu |
| Ohain, Hans Joachim Pabst von ... oil field |
| oil filter ... Ojeda, Alonso de |
| Ojibwa ... Okhwangsangje |
| oki ... Ol Doinyo Lengai |
| Olabimtan, Afolabi ... Old Bridge |
| Old Bulgarian language ... old maid |
| Old Malay language ... Old Town |
| Old Turkic language ... Olduvai Hominid 62 |
| Olduvai Hominid 7 ... oligomictic orthoconglomerate |
| oligonucleotide ... Olives, Mount of |
| Olivet College ... Olsen, Ib Spang |
| Olsen, Kenneth ... Omaha |
| Omaha ... Omiya |
| Omkar temple ... Onassis, Aristotle Socrates |
| Onassis, Jacqueline Kennedy ... Onega Inlet |
| Onega, Lake ... Onomichi |
| Onon River ... Oort, Jan Hendrik |
| oosphere ... open stage |
| open stoping ... opera-comique |
| Opera-Comique ... Ophuls, Max |
| Opie and Anthony ... opposable thumb |
| opposed-piston engine ... optical invariant |
| optical isomerism ... opus interassile |
| opus mixtum ... Orange |
| Orange ... Orban, Viktor |
| Orbe River ... Orchidantha |
| orchiectomy ... Ordinary of the Mass |
| ordinary portland cement ... Oregon System |
| Oregon Trail ... organic geochemistry |
| Organic Law ... Orhon River |
| Orhy ... orienteering |
| orienting response ... Orleanist |
| Orleans ... Ormosia |
| Ormsby-Gore, William George Arthur, 4th Baron Harlech ... Orono, Joseph |
| Orontes River ... Orsino |
| Orsk ... orthometric correction |
| orthomyxovirus ... Oryzomys bolivaris |
| Oryzomys megacephalus ... oscillograph |
| Osco Drug, Inc. ... Oslo Olympics |
| Osman ... Ossa |
| Ossa, Mount ... osteosarcoma |
| osteosclerosis ... Osumi Archipelago |
| Osun ... Otis, Harrison Gray |
| Otis, James ... otterhound |
| Ottingen-Schrattenhofen faience ... Ouchi family |
| Oud, Jacobus Johannes Pieter ... Our Lady of the Assumption Catholic University |
| Our Lady of the Sacred Heart, Daughters of ... outer-belt asteroid |
| outer-sphere reaction ... ovarian ectopic pregnancy |
| ovarian follicle ... overload |
| overlord ... Owachomo Bridge |
| Owain Cyfeiliog ... Oxaeidae |
| oxalate ... oxidative phosphorylation |
| oxide ... Oyampi |
| Oyapock River ... Ozette site |
| Ozick, Cynthia ... |
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