| otalgia | earache |
| OTAN | NATO |
| Otariidae | Eared seal |
| Otawa | Ottawa |
| Otelfingen | Otelfingen |
| Otelo | Othello |
| Otep | OTEP |
| Othello | Othello |
| Other Voices | Other Voices |
| Othmarsingen | Othmarsingen |
| Othniel Charles Marsh | Othniel Charles Marsh |
| Othnielia | Othnielia |
| otimismo | optimism |
| otimismolta | optimism |
| otimista | upbeat |
| otimistalta | cocksure |
| otimizar | optimize |
| Otimização combinatória | Combinatorial optimization |
| Otis Redding | Otis Redding |
| Otite | Ear infection |
| Otite média aguda | Ear infection |
| Otl Aicher | Otl Aicher |
| Otman | Uthman |
| Otniel | Othniel Ben Kenaz |
| Oto da Baviera | Otto of Bavaria |
| Oto I | Otto I |
| Oto I da Grécia | Otto of Greece |
| Otocinclus | Otocinclus |
| Otomis | Otomi people |
| Otori Keisuke | Ōtori Keisuke |
| otorrino | otorhinolaryngologist |
| Otranto | Otranto |
| Otréra | Otrera |
| OTSC | Collective Security Treaty Organisation |
| Ottaviano Nonni | Ottaviano Nonni |
| Ottavio Bottecchia | Ottavio Bottecchia |
| Ottawa | Ottawa |
| Ottawa Senators | Ottawa Senators |
| Ottenhofen | Ottenhofen |
| Ottenhöfen | Ottenhöfen |
| Ottensoos | Ottensoos |
| Ottersweier | Ottersweier |
| Otterup | Otterup municipality |
| Otto Braun | Otto Braun |
| Otto da Grécia | Otto of Greece |
| Otto de Habsburgo | Otto von Habsburg |
| Otto Dix | Otto Dix |
| Otto Finsch | Otto Finsch |
| Otto Friedrich Müller | Otto Friedrich Müller |
| Otto Fritz Meyerhof | Otto Fritz Meyerhof |
| Otto Hahn | Otto Hahn |
| Otto Heinrich Warburg | Otto Heinrich Warburg |
| Otto Jespersen | Otto Jespersen |
| Otto Lilienthal | Otto Lilienthal |
| Otto Ludwig | Otto Ludwig |
| Otto Ludwig Binswanger | Otto Binswanger |
| Otto Mann | Otto Mann |
| Otto Nordenskjöld | Otto Nordenskiöld |
| Otto Paul Hermann Diels | Otto Diels |
| Otto Preminger | Otto Preminger |
| Otto Skorzeny | Otto Skorzeny |
| Otto Stern | Otto Stern |
| Otto von Bismarck | Otto von Bismarck |
| Otto von Guericke | Otto von Guericke |
| Otto Wallach | Otto Wallach |
| Otto Wilhelm Hermann von Abich | Otto Wilhelm Hermann von Abich |
| Otto Wilhelm von Struve | Otto Wilhelm von Struve |
| Ottobrunn | Ottobrunn |
| Ottokar II da Boêmia | Ottokar II of Bohemia |
| Ottorino Respighi | Ottorino Respighi |
| Otumba | Otumba |
| Otus asio | Eastern Screech Owl |
| Otus brucei | Pallid Scops Owl |
| Otus capnodes | Anjouan Scops Owl |
| Otus moheliensis | Moheli Scops Owl |
| Otxandio | Otxandio |
| Otzing | Otzing |
| otária | fur seal |
| otáriomulta | mark |
| Otão | Otho |
| Otólito | Otolith |
| ou | either ... or |
| ou ... ou | either ... or |
| ou então | if not |
| ou exclusivo | xor |
| ou mais | plus |
| ou seja | namely |
| Ouabaína | Ouabain |
| Ouadane | Ouadane |
| Ouagadougou | Ouagadougou |
| Ouargla | Ouargla |
| OUB Centre | OUB Centre |
| Oubeaux | Les Oubeaux |
| Oubritenga | Oubritenga |
| Oudalan | Oudalan |
| Oude-Niedorp | Oude Niedorp |
| Oudenaarde | Oudenaarde |
| Oued Ed-Dahab - Lagouira | Oued Ed-Dahab-Lagouira |
| Oueme | Ouémé Department |
| Ouessant | Ushant |
| Ouest | West |
| ouido | orapple |
| Oulens-sous-Echallens | Oulens-sous-Echallens |
| Oulens-sur-Lucens | Oulens-sur-Lucens |
| Oulipo | Oulipo |
| Oulu | Oulu |
| Oum el Bouaghi | Oum el-Bouaghi |
| Our Lips Are Sealed | Our Lips Are Sealed |
| Our Love To Admire | Our Love to Admire |
| Our Worlds at War | Our Worlds at War |
| Ouran High School Host Club | Ouran High School Host Club |
| Ouranossauro | Ouranosaurus |
| Ourea | Ourea |
| ourela | selvage |
| Ourense | Ourense |
| Ourique | Ourique |
| ourives | goldsmith |
