| Open content | Open content |
| Open di Francia | French Open |
| Open Directory Project | Open Directory Project |
| Open Gaming License | Open Gaming License |
| Open reading frame | Open reading frame |
| Open Sound System | Open Sound System |
| Open source | Open source |
| Open Source Definition | Open Source Definition |
| Open Space Technology | Open Space Technology |
| Open Spectrum | Open spectrum |
| Open Systems Interconnection | OSI model |
| Open Unified Process | OpenUP |
| Open Your Eyes | Open Your Eyes |
| Open Your Heart | Open Your Heart |
| OpenArena | OpenArena |
| OpenCms | OpenCms |
| OpenDocument Format Alliance | OpenDocument Format Alliance |
| OpenLaszlo | OpenLaszlo |
| OpenMosix | OpenMosix |
| OpenNap | Opennap |
| OpenSolaris | OpenSolaris |
| Opensource | Open source |
| OpenSUSE | OpenSUSE |
| OpenVMS | OpenVMS |
| OpenVPN | OpenVPN |
| OpenWRT | OpenWrt |
| opera | work |
| opera a puntate | serial |
| Opera buffa | Opera buffa |
| Opera cementizia | Opus caementicium |
| Opera cinese | Chinese opera |
| opera d'arte | work of art |
| Opera derivata | Derivative work |
| opera di bene | charity |
| opera di carità | charity |
| Opera di Pechino | Beijing opera |
| opera di riassetto dopo una catastrofe | disaster cleanup operation |
| Opera Nazionale Balilla | Opera Nazionale Balilla |
| opera pia | charity |
| Opera reticolata | Opus reticulatum |
| Opera reticulata | Opus reticulatum |
| Opera semiseria | Opera semiseria |
| Opera seria | Opera seria |
| Opera Software | Opera Software |
| operabile | operable |
| Operación Puerto | Operación Puerto doping case |
| operai | labor |
| operaia | worker |
| operaia edile | construction worker |
| operaia metallurgica | metalworker |
| operaia montatrice | fitter |
| operaio | worker |
| operaio edile | construction worker |
| operaio metallurgico | metalworker |
| operaio montatore | fitter |
| operaio specializzato | skilled worker |
