| oscuro | dim |
| OSDL | Open Source Development Labs |
| Osedax | Osedax |
| Osella Corse | Osella |
| Oseltamivir | Oseltamivir |
| Osen | Osen |
| Osečina | Osečina |
| Osgiliath | Osgiliath |
| Oshikoto | Oshikoto Region |
| Oshima | Oshima Province |
| Osho Rajneesh | Osho |
| Oshun | Oshun |
| Osijek | Osijek |
| Osikov | Osikov |
| Osilnica | Osilnica |
| OSINT | Open Source Intelligence |
| Osio di Cordova | Hosius of Córdoba |
| Osip Mandelstam | Osip Mandelstam |
| Osiride | Osiris |
| Osiris | Osiris |
| Oskar Backlund | Oskar Backlund |
| Oskar Dirlewanger | Oskar Dirlewanger |
| Oskar Klein | Oskar Klein |
| Oskar Kokoschka | Oskar Kokoschka |
| Oskar Morgenstern | Oskar Morgenstern |
| Oskar Schindler | Oskar Schindler |
| Oskar Troplowitz | Oscar Troplowitz |
| Oskarshamn | Oskarshamn Municipality |
| Oslany | Oslany |
| Oslip | Oslip |
| Oslo | Oslo |
| Oslofjord | Oslofjord |
| Osman I | Osman I |
| Osman II | Osman II |
| Osman III | Osman III |
| Osmanthus fragrans | Sweet Osmanthus |
| osmio | osmium |
| osmosi | osmosis |
| osmosi inversa | reverse osmosis |
| osmotico | osmotic |
| Osnabrück | Osnabrück |
| Osnago | Osnago |
| Osogna | Osogna |
| Osorkon B | Osorkon III |
| Osoto Gari | Osoto Gari |
| ospedale | clinic |
| ospedale da campo | field hospital |
| Ospedale di AHEPA | AHEPA University Hospital |
| Ospedale di Sant Pau | Hospital de Sant Pau |
| ospedale psichiatrico | mental hospital |
| ospedalizzare | hospitalize |
| ospedalizzazione | hospitalization |
