| only half a | parcial |
| only just | apenas |
| only just in time | justo a tiempo |
| Onmyo-Za | Onmyouza |
| Onnens | Onnens |
| ONO Estadi | ONO Estadi |
| Onofre Marimón | Onofre Marimón |
| Onoin | Allium cepa |
| Onomacritus | Onomácrito |
| Onomastica | Onomástica |
| Onomastics | Onomástica |
| onomatopoeia | onomatopeya |
| onomatopoeic | onomatopéyico |
| Ononis | Ononis |
| Ononis spinosa | Ononis spinosa |
| Onopordum | Onopordum |
| Onotoa | Onotoa |
| onrush | arremetida |
| Onryō | Onryō |
| ONS | Onza |
| Ons Hémécht | Ons Hémécht |
| Onsen | Aguas termales |
| Onsernone | Onsernone |
| onset | principio |
| onslaught | ataque violento |
| Ontario | Ontario |
| Ontinyent | Onteniente |
| onto | encima de |
| ontogenesis | ontogénesis |
| ontogeny | ontogénesis |
| Ontological argument | Argumento ontológico |
| ontology | ontología |
| OntoMeeting | OntoMeeting |
| Ontronik Khachaturian | Andy Khachaturian |
| Onuphrius | Onofre |
| onus | carga |
| onward | adelante |
| onwards | adelante |
| Onyar | Oñar |
| Onychomycosis | Tinea unguiüm |
| onyx | ónice |
| Onyx Grand Prix | Onyx |
| Onyx River | Onyx |
| Oodaaq | Oodaaq |
| Oogamy | Oogamia |
| OOL | Lenguaje de objetos |
| Oola | Oola |
| Oology | Oología |
| Oomiac | Umiak |
| Oomiak | Umiak |
| Oomoto | Oomoto |
| Oompa-Loompa | Oompa Loompa |
| Oomycete | Oomycota |
| Ooogamy | Oogamia |
| Oort cloud | Nube de Oort |
| Oort constant | Constante de Oort |
| Oort formula | Fórmula de Oort |
| Oostende | Ostende |
| Oosterhoff groups | Grupo de Oosterhoff |
| Oosterpark Stadion | Oosterpark Stadion |
| Ootacamund | Udhagamandalam |
| Ootheca | Ooteca |
| Oozaru | Ōzaru |
| ooze | filtrarse |
| oozy | legamoso |
| OP | Orden de los Hermanos Predicadores |
| Opabinia | Opabinia |
| OPAC | OPAC |
| opacity | opacidad |
| opal | ópalo |
| opalescence | opalescencia |
| opalescent | opalescente |
| opaque | opaco |
| opaqueness | impenetrabilidad |
| Opata people | Ópata |
| Opava | Opava |
| Opava District | Opava |
| Opava River | Opava |
| OPEC | OPEP |
