| Ottawa | Ottawa |
| Ottawa River | Ottawa |
| otter | vidra |
| Otto Fritz Meyerhof | Otto Fritz Meyerhof |
| Otto Hahn | Otto Hahn |
| Otto Herschmann | Otto Herschmann |
| Otto Hölder | Otto Ludwig Hölder |
| Otto I | Oton I. Veliki |
| Otto Stern | Otto Stern |
| Otto Struve | Otto Struve |
| Otto von Bismarck | Otto von Bismarck |
| Otto von Guericke | Otto von Guericke |
| Otto Wilhelm Hermann von Abich | Otto Wilhelm Hermann von Abich |
| Otto Wilhelm von Struve | Otto Vasiljevič Struve |
| Ottoman | Turčija |
| Ottoman Empire | Osmansko cesarstvo |
| Ottoman Greece | Grčija |
| Ottoman Turks | Turki |
| Ottorino Respighi | Ottorino Respighi |
| Ouagadougou | Ouagadougou |
| Ouest Department | Zahod |
| Oulton Park International Gold Cup | International Gold Cup |
| Ounce | Leopard |
| our | naš |
| Our Lady of the Rocks | Gospa od Škrpjela |
| Our River | Our |
| Out here | Tuka |
| Out of doubt | Najbrž |
| Out of the world | Nenavaden |
| Outer planet | Saturn |
| outer space | neurbanizirani prostor |
| Outlet tower | Redovnik |
| Outremer | Sveta dežela |
| Outubro | Oktober |
| Oval tube station | Oval |
| Ovally | Oval |
| ovary | jajčnik |
| oven | peč |
| overburden | krovna plast |
| overexploitation | čezmerno izkoriščanje |
| overfertilisation | čezmerno gnojenje |
| overfishing | čezmeren ribolov |
| overflow | preliv |
| overflowing | poplava |
| overgrazing | čezmerna paša |
| overhead power line | daljnovod |
| Overlying stratum | Krov |
| overpopulation | prenaseljenost |
| overturn | obratna termična plastovitost |
| Overview | Predogled |
| overwintering | prezimovanje |
| Ovid | Publij Ovidij Naso |
| ovine | ovca |
| Ovis | Domača ovca |
| Ovum | Jajce |
| Owen | Owen |
| Owen Chamberlain | Owen Chamberlain |
| Owen Gun | Owen |
| Owen Willans Richardson | Owen Willans Richardson |
| Owerpass | Brv |
| owl | sova |
| Owner of an estate | Naboj |
| ownership | lastništvo |
| ox | vol |
| Oxford | Oxford |
| Oxfordian stage | Oxford |
| oxidation | oksidacija |
| oxidation-reduction | oksidacija-redukcija |
| oxide | oksid |
| oxidisable material | snov |
| oxidising agent | oksidant |
| oxygen | kisik |
| oxygen content | vsebnost kisika |
| Oxygen cylinder | Kisik |
| oxygenation | okisičenje |
| Oxygene | Kisik |
| oyster | ostrige |
| oyster farming | gojenje ostrig |
| Oyster sauce | Omaka iz ostrig |
