| ondear | ondeggiare |
| Ondearse | Onda |
| ondeo | sventolio |
| Ondium Martenot | Onde Martenot |
| ondulación | onda |
| ondulado | ondulato |
| ondular | increspare |
| Ondárroa | Ondarroa |
| One | One |
| One by One | One by One |
| One Day In Your Life | One Day in Your Life |
| One Piece | One Piece |
| One Tree Hill | One Tree Hill |
| One-hit wonder | One-hit wonder |
| Onee-sama | Onēsama |
| Onegai Teacher | Onegai Teacher |
| Onegai Twins | Onegai Twins |
| Oneglia | Oneglia |
| Oneida | Oneida |
| oneroso | oneroso |
| Oneworld | Oneworld |
| Onex | Onex |
| Oni | The Faculty |
| Oniromancia | Oniromanzia |
| Oniros | Oneiroi |
| Onix | Onix |
| ONL | ORL |
| Online | Online |
| Online Certificate Status Protocol | Online Certificate Status Protocol |
| Onnens | Onnens |
| Onofre Marimón | Onofre Marimon |
| onomatopeya | onomatopea |
| Onomástica | Onomastica |
| Onomástica árabe | Onomastica araba |
| Ononis spinosa | Ononis spinosa |
| Ons Hémécht | Ons Heemecht |
| Onsernone | Onsernone |
| Onslaught | Onslaught |
| Ontario | Ontario |
| Onteniente | Ontinyent |
| ontogénesis | ontogenesi |
| ontología | ontologia |
| OntoMeeting | OntoMeeting |
| ONU | Nazioni Unite |
| Onychophora | Onychophora |
| Onyx | Onyx |
| onza | oncia |
| onza bermeja | puma |
| Onza líquida | Oncia liquida |
| Onán | Onan |
| Oocito | Ovocita |
| Oodaaq | Oodaaq |
| Oogamia | Ooogamia |
| Oompa Loompa | Umpa Lumpa |
| Oomycota | Oomycota |
| Ooo | Traghetto |
| Oopart | OOPArt |
| Oosterbeek | Renkum |
| Ooteca | Ooteca |
| Op-art | Op art |
| OPAC | OPAC |
| opacidad | oscurità |
| Opacidad de Kramers | Opacità di Kramers |
| Opacidad de Rosselang | Opacità di Rosseland |
| opaco | pallido |
| opalescencia | opalescenza |
| opalescente | opalescente |
| Opava | Opava |
| opcional | facoltativo |
