| onorevole | Abgeordnete |
| onori militari | militärische Ehren |
| onorificenza | Preis |
| onorifico | ehrend |
| Onorio | Flavius Honorius |
| ONS | ONS |
| Ons Heemecht | Ons Heemecht |
| Onsen | Onsen |
| Onsernone | Onsernone TI |
| Onslaught | Angriff |
| onta | Schande |
| ontano | Erle |
| ontano nero | Schwarzerle |
| Ontario | Ontario |
| Ontelaunee Township | Ontelaunee Township |
| ontogenesi | Ontogenese |
| ontologia | Ontologie |
| ontologico | ontologisch |
| OntoMeeting | OntoMeeting |
| ONU | UNO |
| ONUB | Opération des Nations Unies au Burundi |
| ONUC | Opération des Nations Unies au Congo |
| ONUMOZ | Opération des Nations Unies au Mozambique |
| Onuris | Anhor |
| Onychophora | Stummelfüßer |
| Onyx | Onyx |
| OO | OO |
| Oocito | Eizelle |
| Oodaaq | Oodaaq |
| Ooliti | Oolith |
| Oomycota | Eipilze |
| Oost-Cappel | Oost-Cappel |
| OP | OP |
| Op art | Op-Art |
| OPA | Opa |
| OPAC | Online Public Access Catalogue |
| opacità | Undurchsichtigkeit |
| opaco | matt |
| opale | Opal |
| opalescente | opalisierend |
| opalescenza | Opalisieren |
| Opava | Opava |
| OPEC | OPEC |
| Opel | Opel |
| Opel Antara | Opel Antara |
| Opel Corsa | Opel Corsa |
| Opel Frontera | Opel Frontera |
| Opel Meriva | Opel Meriva |
| Opel Vectra | Opel Vectra |
| Opel Zafira | Opel Zafira |
| Open content | Freie Inhalte |
| Open di Francia | French Open |
| Open Directory Project | Open Directory Project |
| Open Gaming License | Open Gaming License |
| Open reading frame | Offener Leserahmen |
| Open Shortest Path First | Open Shortest Path First |
| Open Sound System | Open Sound System |
| Open source | Open Source |
| Open Source Definition | Open Source Definition |
| Open Space Technology | Open Space |
| Open Systems Interconnection | OSI-Modell |
| Open Unified Process | Open Unified Process |
| Open Water | Open Water |
| OpenArena | OpenArena |
| OpenCms | OpenCms |
| OpenDocument Format Alliance | Open Document Format Alliance |
| OpenLaszlo | OpenLaszlo |
| OpenNap | OpenNap |
| OpenPGP | OpenPGP |
| OpenSolaris | OpenSolaris |
| Opensource | Open Source |
| OpenSUSE | OpenSuse |
| OpenVMS | Virtual Memory System |
| OpenVPN | OpenVPN |
| OpenWRT | OpenWrt |
| opera | Stück |
| opera a puntate | Serie |
| Opera buffa | Opera buffa |
| Opera cinese | Chinesische Oper |
| opera d'arte | Kunststück |
| Opera dei Pupi | Sizilianisches Marionettentheater |
| Opera derivata | Bearbeitung |
| opera di bene | Hilfswerk |
