| OS X Mountain Lion | Apple Inc. | 2012 | Apple Public Source License |
| OS X Yosemite | Apple Inc. | 2014 | Apple Public Source License |
| OS-9 | Microware | — | — |
| Os.bf | Brainfuck Enterpise Solutions | — | WTFPL |
| OS/2 | IBM, Microsoft | — | proprietary license |
| OS/2 Warp 4 | IBM | 1996 | — |
| OS/2 Warp Server | IBM | — | — |
| OS/360 | IBM | 1966 | proprietary license |
| OS/390 | IBM | 1995 | — |
| OS/400 | IBM | — | — |
| OS/8 | Digital Equipment Corporation | — | — |
| OS/VS1 | IBM | — | — |
| OS/VS2 | IBM | 1972 | — |
| OS/VX | — | — | GNU General Public License |
| OS2000 | Scientific Research Institute of System Development | — | — |
| OS4000 | GEC Computers | 1977 | — |
| OSASK | — | — | — |
| OSEck | ENEA AB | — | — |
| OSF/1 | Open Software Foundation | — | — |
| OsFree | — | — | — |
| OSGeoLive | — | — | — |
| OSIAN | — | — | BSD licenses |
| OSIV/MSP | Fujitsu | — | — |
| OSL | Moscow Center of SPARC Technologies | — | — |
| OSMC | — | — | GNU General Public License, version 2.0 |
| OSv | — | — | BSD licenses |
| OSx86 | — | — | — |
| Otonomous Operating System | — | — | — |
| OxygenOS | OnePlus | 2015 | — |
| OZONE | — | — | GNU General Public License |
| PAC-ROM | — | 2012 | — |
| Paipix | — | — | — |
| Paldo GNU/Linux | Jürg Billeter | — | GNU General Public License |
| Palm OS | Palm, Inc., Access | — | proprietary license |
| PAN-OS | Palo Alto Networks | — | — |
| PANIX | NEC | — | — |
| Panos | Acorn Computers | — | — |
| Parabola GNU/Linux-libre | — | — | GNU General Public License |
| Paranoid Android | — | 2012 | — |
| Parch Linux | — | — | GNU General Public License |
| Pardus | TÜBİTAK | — | GNU General Public License |
| Pardus 1.0 | TÜBİTAK | 2005 | — |
| Parrot OS | — | — | — |
| Parsix | — | — | — |
| Parted Magic | — | — | GNU General Public License |
| PascalDOS | — | — | — |
| PC-MOS/386 | The Software Link | — | GNU General Public License |
| PC-UX | NEC | 1984 | — |