| once | eleven |
| once de noviembre | Veteran's Day |
| Once in a LIVEtime | Once in a LIVEtime |
| Once minutos | Eleven Minutes |
| onceavo | eleventh |
| Oncifelis | Oncifelis |
| Oncifelis guigna | Kodkod |
| oncología | oncology |
| Oncorhynchus | Oncorhynchus |
| Oncovirus | Oncovirus |
| onda | wave |
| onda acústica | sound wave |
| onda alpha | alpha wave |
| onda beta | beta wave |
| onda corta | shortwave |
| Onda de Alfven | Alfven wave |
| onda de Alfvén | Alfvén wave |
| onda de choque | shock wave |
| Onda de choque en forma de ar | Bow shock wave |
| Onda de choque ionizane | Ionizing shock wave |
| onda de Elliott | Elliott wave |
| Onda de plasma | Plasma wave |
| Onda de Rossby | Rossby wave |
| onda dirigida | beam |
| onda electromagnética | electromagnetic wave |
| Onda electrostática | Electrostatic wave |
| Onda en el plasma iónico | Ion plasma wave |
| onda expansiva | shock wave |
| onda gravitacional | gravitational wave |
| Onda gravitatoria | Gravitational waves |
| Onda hidromagnética | Hydromagnetic wave |
| Onda mecánica | Mechanical wave |
| Onda milimétrica | Millimeter wave |
| Onda periódica | Periodic function |
| Onda radio | Radiowave |
| onda senoidal | sine wave |
| onda sonora | sound wave |
| onda theta | theta wave |
| Onda triangular | Triangle wave |
| onda verde | green wave |
| Ondangua | Ondangwa |
| Ondas Martenot | Ondes Martenot |
| Ondas musicales | Ondes Martenot |
| onde | whither |
| ondeante | billowy |
| ondear | curl |
| ondeo | flapping |
| ondina | undine |
| Ondium Martenot | Ondes Martenot |
| Ondol | Ondol |
| ondulación | wave |
| ondulado | grooved |
| ondular | curl |
| ondularse | wave |
| Ondárroa | Ondarroa |
| One | One Railway |
| One Canada Square | One Canada Square |
| One Child | One Child |
| One Day In Your Life | One Day in Your Life |
| One Piece | One Piece |
| One Pound Gospel | One-Pound Gospel |
| One Tree Hill | One Tree Hill |
| Onee-sama | Oneesama |
| Oneglia | Oneglia |
| Oneida | Oneida tribe |
| oneroso | onerous |
| Oneworld | Oneworld |
| Oneşti | Oneşti |
| Onironauta | Oneirology |
| Oniros | Oneiroi |
| ONL | ENT |
| Online Computer Library Center | OCLC |
| Onmyouza | Onmyo-Za |
| Onnens | Onnens |
| ONO Estadi | ONO Estadi |
| Onofre | Onuphrius |
| Onofre Marimón | Onofre Marimón |
