| Ontario | Ontario |
| Ontario Hockey League | Ontario Hockey League |
| ontogenèse | ontogeneza |
| ontologie | ontologia |
| ontologique | ontologiczny |
| ONU | ONZ |
| Onychogalea | Pazurogon rudopręgi |
| Onychogalea lunata | Pazurogon księżycowy |
| Onychophora | Pazurnice |
| onze | jedenaście |
| Oolithe | Oolity |
| Oologie | Owologia |
| Oomycota | Lęgniowce |
| Oost-Cappel | OoSaint-Cappel |
| OP | Dominikanie |
| Op Art | Op-art |
| Opabinia | Opabinia |
| OPAC | OPAC |
| Opale | Opal |
| Opatija | Opatija |
| Opava | Opawa |
| Opchanacanough | Opchanacanough |
| Opel | Adam Opel AG |
| Opel Astra G | Opel Astra |
| Opel Astra H | Opel Astra |
| Opel Corsa | Opel Corsa |
| Opel Meriva | Opel Meriva |
| Opel Vectra | Opel Vectra |
| Opel Zafira | Opel Zafira |
| Open | Nierozstrzygnięte |
| Open Arena | OpenArena |
| Open Clip Art Library | Wolna biblioteka klipartów |
| Open database connectivity | ODBC |
| Open de Bangalore | Sony Ericsson International |
| Open de Barcelone | Open Seat |
| Open de Bois-le-Duc | Ordina Open |
| Open de Bombay | Kingfisher Airlines Tennis Open |
| Open de Bâle | Davidoff Swiss Indoors Basel |
| Open de Chennai | Chennai Open |
| Open de Chine | China Open |
| Open de Corée | Hansol Korea Open Tennis Championships |
| Open de Croatie Umag | Studena Croatia Open Umag |
| Open de Doha | Qatar ExxonMobil Open |
| Open de Dubaï | Dubai Tennis Championships |
| Open de France de badminton | French Open |
| Open de Gstaad | Allianz Suisse Open Gstaad |
| Open de Halle | Gerry Weber Open |
| Open de Las Vegas | Tennis Channel Open |
| Open de Los Angeles | East West Bank Classic |
| Open de Moselle | Open de Moselle |
| Open de Munich | BMW Open |
| Open de Nottingham | The Nottingham Open |
| Open de Poertschach | Hypo Group Tennis International |
| Open de Prague | ECM Prague Open |
| Open de Roumanie | BCR Open Romania |
| Open de San José | SAP Open |
| Open de Sicile | Campionati Internazionali di Sicilia |
| Open de Slovénie | Banka Koper Slovenia Open |
| Open de Sopot | Orange Prokom Open |
| Open de Stockholm | If Stockholm Open |
| Open de Suède | Catella Swedish Open |
| Open de Tachkent | Tashkent Open |
| Open de Tel Aviv | Anda Open |
| Open de Thaïlande | Thailand Open |
| Open de Tōkyō | Toray Pan Pacific Open |
| Open de Valence | Open de Tenis Comunidad Valenciana |
| Open de Viña del Mar | Movistar Open |
| Open de Zurich | Zurich Open |
| Open Directory Project | Open Directory Project |
| Open du Brésil | Brasil Open |
| Open du Canada | Rogers Cup |
| Open du Japon | AIG Japan Open Tennis Championships |
| Open du Mexique | Abierto Mexicano TELCEL |
| Open Gaz de France | Open Gaz de France |
| Open Group | The Open Group |
| Open invention network | Open Invention Network |
| Open shortest path first | OSPF |
| Open Sound System | Open Sound System |
| Open Source | Otwarte oprogramowanie |
| Open Source Definition | Definicja Otwartego Źródła |
| Open Source Development Labs | Open Source Development Labs |
| Open University | Open University |
| Open XML | Office Open XML |
| Open your eyes | Open Your Eyes |
| Open Your Heart | Open your Heart |
| OpenGraphics | Open Graphics Project |
| OpenLaszlo | OpenLaszlo |
| OpenMosix | OpenMosix |
| OpenPGP | OpenPGP |
| OpenSolaris | OpenSolaris |
| OPENSTEP | OpenStep |
| OpenVPN | OpenVPN |
| OpenWrt | OpenWrt |
| OPEP | OPEC |
| Opera | Opera |
| Opera Garnier | Opera Garnier |
| Opera seria | Opera seria |
| Opera Software | Opera Software |
| Operación Triunfo | Star Academy |
| Operation Barbarossa | Plan Barbarossa |
| Operation Ivy | Operation Ivy |
| Operation Wolf | Operation Wolf |
| OPEX | Opex |
| Ophel | Ofel |
| Ophichthidae | Żmijakowate |
| Ophidiiformes | Wyślizgokształtne |
| Ophioglossaceae | Nasięźrzał |
| Ophioglosse commun | Nasięźrzał pospolity |
| Ophioglossum | Nasięźrzał |
| Ophiolite | Ofiolit |
| Ophion | Ofion |
| Ophiuchus | Wężownik |
| Ophiuroidea | Wężowidła |
| ophtalmologie | okulistyka |
| Ophtalmoscope | Oftalmoskop |
