| onomatopeya | onomatopoeia |
| onomatopéyico | onomatopoeic |
| Onomácrito | Onomacritus |
| Onomástica | Onomastics |
| Onomástica china | Chinese name |
| Onomástica japonesa | Japanese name |
| Onomástica árabe | Arabic name |
| Ononis | Ononis |
| Ononis spinosa | Ononis spinosa |
| Onopordum | Onopordum |
| Onotoa | Onotoa |
| Onryō | Onryō |
| Ons Hémécht | Ons Hémécht |
| Onsernone | Onsernone |
| Ontario | Ontario |
| Onteniente | Ontinyent |
| ontogénesis | ontogenesis |
| ontología | ontology |
| OntoMeeting | OntoMeeting |
| ONU | United Nations |
| onza | ounce |
| onza bermeja | cougar |
| Onza de África | Cheetah |
| onza troy | troy ounce |
| Onán | Onan |
| Onésimo | Onesimus |
| Oodaaq | Oodaaq |
| Oogamia | Oogamy |
| Oola | Oola |
| Oología | Oology |
| Oomoto | Oomoto |
| Oompa Loompa | Oompa-Loompa |
| Oomycota | Oomycete |
| Ooo | Ferry |
| Oopart | Out-of-place artifact |
| Oosterbeek | Renkum |
| Oosterpark Stadion | Oosterpark Stadion |
| Ooteca | Ootheca |
| Opabinia | Opabinia |
| OPAC | OPAC |
| opacidad | impenetrability |
| opaco | dead |
| opalescencia | opalescence |
| opalescente | opalescent |
| Opava | Opava |
| OPC | OLE for process control |
| opcion | option |
| opcional | facultative |
| opción | option |
| opción de compra | call |
| opción de venta | put |
| opción disponible | available choice |
| Opel | Opel |
| Opel Astra | Opel Astra |
| Opel Corsa | Opel Corsa |
| Opel Kadett | Opel Kadett |
| Opel Meriva | Opel Meriva |
| Opel Vectra | Opel Vectra |
| Opel Zafira | Opel Zafira |
| Open Ajax | OpenAjax Alliance |
| Open Clip Art Library | Open Clip Art Library |
| Open Database Connectivity | Open Database Connectivity |
| Open de Tenis Comunidad Valenciana | Open de Tenis Comunidad Valenciana |
| Open Directory Project | Open Directory Project |
| Open Educational Resources | Open educational resources |
| Open Relay | Open mail relay |
| Open Your Heart | Open Your Heart |
| Open-source Ticket Request System | OTRS |
| Openarena | OpenArena |
| Openbox | Openbox |
| OpenCanvas | OpenCanvas |
| OpenCms | OpenCms |
| OpenCourseWare | MIT OpenCourseWare |
| OpenNap | Opennap |
| Opensolaris | OpenSolaris |
| OpenSound Control | OpenSound Control |
| OpenSUSE | OpenSUSE |
| OpenVMS | OpenVMS |
| OpenVPN | OpenVPN |
| OPEP | OPEC |
| Opera | Opera |
| Opera buffa | Opera buffa |
| Opera Game | Opera game |
| Opera Software | Opera Software |
| operable | feasible |
