| .OBI | Microsoft Corporation | Arquivos de configuração | Outlook RSS Subscription File | Microsoft Outlook 2016 | Microsoft Corporation |
| .OBT | Open Source | Arquivos de configuração | Openbox Theme File | Openbox | Open Source |
| .OCE | Oracle Corporation | Arquivos de configuração | Open Catalog Extension Connection File | Oracle Business Intelligence Suite | Oracle Corporation |
| .ODC | Microsoft Corporation | Arquivos de configuração | Office Data Connection File | Microsoft Excel | Microsoft Corporation |
| .OEM | Microsoft Corporation | Arquivos de configuração | Original Equipment Manufacturer Setup File | Microsoft Windows | Microsoft Corporation |
| .OFD | Cypresslogic Systems Inc. | Arquivos de configuração | ObjectView Form Definition Data | ObjectView | Cypresslogic Systems Inc. |
| .OFFICEUI | Microsoft Corporation | Arquivos de configuração | Microsoft Office UI Customization File | Microsoft Office | Microsoft Corporation |
| .OFL | Ots Labs | Arquivos de configuração | OtsTurntables Playlist | OtsTurntables | Ots Labs |
| .OIF | OriginLab | Arquivos de configuração | Origin Import Filter File | OriginLan Origin Viewer | OriginLab |
| .OMS | Hewlett-Packard | Arquivos de configuração | HP Printer Substrate Presets Package File | HP Embedded Web Server | Hewlett-Packard |
| .ONETOC | Microsoft Corporation | Arquivos de configuração | Microsoft OneNote Table Of Contents | Microsoft OneNote | Microsoft Corporation |
| .ONETOC2 | Microsoft Corporation | Arquivos de configuração | Microsoft OneNote Table Of Contents File | Microsoft OneNote | Microsoft Corporation |
| .OODGUISETTINGS | O&O Software | Arquivos de configuração | O&O Defrag GUI Settings | O&O Defrag | O&O Software |
| .OPS | Microsoft Corporation | Arquivos de configuração | Office Profile Settings File | Microsoft Office | Microsoft Corporation |
| .OPT | LexisNexis | Arquivos de configuração | Concordance Database File | LexisNexis Concordance | LexisNexis |
| .OSDX | Microsoft Corporation | Arquivos de configuração | Search Connector Description File | Microsoft Windows | Microsoft Corporation |
| .OSS | Microsoft Corporation | Arquivos de configuração | Microsoft Outlook Saved Search File | Microsoft Outlook | Microsoft Corporation |
| .OTH | Apache Software Foundation | Arquivos de configuração | OpenDocument HTML Template | Apache OpenOffice | Apache Software Foundation |
| .OTZ | OpenLP Team | Arquivos de configuração | OpenLP Theme File | OpenLP | OpenLP Team |
| .OVPN | OpenVPN Technologies | Arquivos de configuração | OpenVPN Configuration File | OpenVPN | OpenVPN Technologies |
| .OXO | Open Source | Arquivos de configuração | Ox Object-oriented Matrix Programming Language Source Code | OX Professional | Open Source |
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