| Extension | File Description |
| .ORB | Uncommon Files |
| .ORC | Audio Files |
| .ORCA | Uncommon Files |
| .ORD | Uncommon Files |
| .ORE | Executable Files |
| .ORF | Raster Image Files |
| .ORG | Data Files |
| .ORGAN | Uncommon Files |
| .ORH | Uncommon Files |
| .ORI | Backup Files |
| .ORIG | Backup Files |
| .ORIGINAL | Uncommon Files |
| .ORJ | Uncommon Files |
| .ORK | Uncommon Files |
| .ORL | Uncommon Files |
| .ORM | Uncommon Files |
| .ORMV | Uncommon Files |
| .ORN | Uncommon Files |
| .ORO | Uncommon Files |
| .ORP | Uncommon Files |
| .ORQ | Web Files |
| .ORR | Uncommon Files |
| .ORS | Web Files |
| .ORT | Uncommon Files |
| .ORU | Uncommon Files |
| .ORV | Uncommon Files |
| .ORW | Uncommon Files |
| .ORX | Uncommon Files |
| .ORY | Uncommon Files |
| .ORZ | Uncommon Files |
| .OS | Developer Files |
| .OS0 | Uncommon Files |
| .OS1 | Uncommon Files |
| .OS2 | Uncommon Files |
| .OS3 | Uncommon Files |
| .OS4 | Uncommon Files |
| .OS5 | Uncommon Files |
| .OS6 | Uncommon Files |
| .OS7 | Uncommon Files |
| .OS8 | Uncommon Files |
| .OS9 | Uncommon Files |
| .OSA | Uncommon Files |
| .OSB | GIS Files |
| .OSC | GIS Files |
| .OSD | Uncommon Files |
| .OSDX | Settings Files |
| .OSE | Uncommon Files |
| .OSF | Uncommon Files |
| .OSFPVG | Data Files |
| .OSG | Uncommon Files |
| .OSH | Uncommon Files |
| .OSI | Uncommon Files |
| .OSJ | Uncommon Files |
| .OSK | Uncommon Files |
| .OSL | Uncommon Files |
| .OSM | Uncommon Files |
| .OSMALLER | Uncommon Files |
| .OSN | Uncommon Files |
| .OSO | Uncommon Files |
| .OSp | Uncommon Files |
| .OSQ | Uncommon Files |
| .OSR | Uncommon Files |
| .OSS | Settings Files |
| .OST | Misc Files |
| .OSTANDARD | Uncommon Files |
| .OSU | Uncommon Files |
| .OSV | Uncommon Files |
| .OSW | Uncommon Files |
| .OSX | Uncommon Files |
| .OSY | Uncommon Files |
| .OSZ | Uncommon Files |
| .OT | Data Files |
| .OT0 | Uncommon Files |
| .OT1 | Uncommon Files |
| .OT2 | Uncommon Files |
| Extension | File Description |
| .OT3 | Uncommon Files |
| .OT4 | Uncommon Files |
| .OT5 | Uncommon Files |
| .OT6 | Uncommon Files |
| .OT7 | Uncommon Files |
| .OT8 | Uncommon Files |
| .OT9 | Uncommon Files |
| .OTA | Uncommon Files |
| .OTB | Raster Image Files |
| .OTC | Misc Files |
| .OTD | Uncommon Files |
| .OTE | Uncommon Files |
| .OTF | Font Files |
| .OTG | Vector Image Files |
| .OTH | Web Files |
| .OTHERFOLDE | Uncommon Files |
| .OTHERS | Uncommon Files |
| .OTI | Raster Image Files |
| .OTJ | Uncommon Files |
| .OTK | Uncommon Files |
| .OTL | Data Files |
| .OTM | Executable Files |
| .OTM2 | Uncommon Files |
| .OTN | Uncommon Files |
| .OTO | Uncommon Files |
| .OTOSET | Uncommon Files |
| .OTP | Data Files |
| .OTQ | Uncommon Files |
| .OTR | Uncommon Files |
| .OTRKEY | Uncommon Files |
| .OTS | Spreadsheet Files |
| .OTT | Text Files |
| .OTU | Uncommon Files |
| .OTV | Uncommon Files |
| .OTW | Uncommon Files |
| .OTX | Data Files |
| .OTY | Uncommon Files |
| .OTZ | Uncommon Files |
| .OU! | Uncommon Files |
| .OU0 | Uncommon Files |
| .OU1 | Uncommon Files |
| .OU2 | Uncommon Files |
| .OU3 | Uncommon Files |
| .OU4 | Uncommon Files |
| .OU5 | Uncommon Files |
| .OU6 | Uncommon Files |
| .OU7 | Uncommon Files |
| .OU8 | Uncommon Files |
| .OU9 | Uncommon Files |
| .OUA | Uncommon Files |
| .OUB | Uncommon Files |
| .OUC | Uncommon Files |
| .OUD | Uncommon Files |
| .OUE | Uncommon Files |
| .OUF | Uncommon Files |
| .OUG | Uncommon Files |
| .OUH | Uncommon Files |
| .OUI | Uncommon Files |
| .OUJ | Uncommon Files |
| .OUK | Uncommon Files |
| .OUL | Uncommon Files |
| .OUM | Uncommon Files |
| .OUN | Uncommon Files |
| .OUO | Uncommon Files |
| .OUP | Uncommon Files |
| .OUQ | Uncommon Files |
| .OUR | Uncommon Files |
| .OUS | Uncommon Files |
| .OUT | Executable Files |
| .OUTPUT | Uncommon Files |
| .OUU | Uncommon Files |
| .OUV | Uncommon Files |
| .OUW | Uncommon Files |
| .OUX | Uncommon Files |
| .OUY | Uncommon Files |
