| Opel Vectra | Opel Vectra |
| Opel Zafira | Opel Zafira |
| Open Directory Project | Open Directory Project |
| Open Shortest Path First | Open Shortest Path First |
| Open Sound System | Open Sound System |
| Open Source Development Labs | OSDL |
| Open Systems Interconnection | Open Systems Interconnection |
| Open Water | Open Water |
| OpenArena | OpenArena |
| OpenSolaris | OpenSolaris |
| OpenSUSE | OpenSUSE |
| OpenVMS | OpenVMS |
| OpenVPN | OpenVPN |
| opera | opera |
| Opera -n | Opera |
| Opera buffa | Opera buffa |
| Opera metal | Symphonic metal |
| opera- | lirico |
| operabel | operabile |
| Operación Puerto | Operación Puerto |
| operation | chirurgia |
| Operation -en -er | Operazione unaria |
| Operation Ajax | Operazione Ajax |
| Operation Bagration | Operazione Bagration |
| Operation Barbarossa | Operazione Barbarossa |
| Operation Blå | Operazione Blu |
| Operation Condor | Operazione Condor |
| Operation Corkscrew | Operazione Corkscrew |
| Operation Desert Storm | Operazione Desert Storm |
| Operation Dynamo | Operazione Dinamo |
| Operation Entebbe | Operazione Entebbe |
| Operation Flashpoint | Operation Flashpoint |
| Operation Frühlingserwachen | Operazione Frühlingserwachen |
| Operation Greif | Operazione Greif |
| Operation Husky | Operazione Husky |
| Operation Ivy | Operation Ivy |
| Operation kjoltyg | Operazione sottoveste |
| Operation Ladbroke | Operazione Ladbroke |
| Operation Market Garden | Operazione Market Garden |
| Operation Merkur | Battaglia di Creta |
| Operation Neptune | Operazione Nettuno |
| Operation Nordwind | Operazione Nordwind |
| Operation Northwoods | Operazione Northwoods |
| Operation Overlord | Sbarco in Normandia |
| Operation Reinhard | Operazione Reinhard |
| Operation Rösselsprung | Operazione Rösselsprung |
| Operation Seelöwe | Operazione Seelöwe |
| Operation Sommarregn | Operazione Piogge Estive |
| Operation Tannenberg | Operazione Tannenberg |
| Operation Torch | Operazione Torch |
| Operation Weserübung | Operazione Weserübung |
| Operations Support System | Operational Support Systems |
| Operationsförstärkare | Amplificatore operazionale |
| operationssal | sala operatoria |
| Operationssystem | Sistema operativo |
| operativsystem | sistema operativo |
| Operativsystemkärna | Kernel |
| Operatorprioritet | Ordine delle operazioni |
