| OSASK | — | — | — |
| OsFree | — | — | — |
| OSIAN | — | — | BSD licenses |
| OSIV/MSP | Fujitsu | — | — |
| OSL | Moscow Center of SPARC Technologies | — | — |
| OSMC | — | — | GNU General Public License, version 2.0 |
| OSv | — | — | BSD licenses |
| OSx86 | — | — | — |
| OxygenOS | OnePlus | 2015 | — |
| OZONE | — | — | GNU General Public License |
| Paipix | — | — | — |
| Palm OS | Palm, Inc., Access | — | proprietary license |
| PAN-OS | Palo Alto Networks | — | — |
| Panos | Acorn Computers | — | — |
| Pardus 1.0 | TÜBİTAK | 2005 | — |
| Parsix | — | — | — |
| PascalDOS | — | — | — |
| PC-MOS/386 | The Software Link | — | GNU General Public License |
| PC-UX | NEC | 1984 | — |
| PDOS | — | — | — |
| PelicanHPC | — | — | — |
| PenDOS | Communication Intelligence Corporation (United States) | — | — |
| Persia OS | — | 2024 | — |
| Phantom OS | — | — | GNU Lesser General Public License, version 3.0 |
| Phoenix OS | — | — | — |
| Phoenix-RTOS | — | — | — |
| Pi-KVM | — | — | GNU General Public License, version 3.0 |
| Pick operating system | — | — | — |
| PICO OS | PICO | — | — |
| Pilot | Palo Alto Research Center | — | — |
| PiPlay | — | — | — |
| PixelExperience | — | 2019 | — |
| Playdate OS | Panic | — | — |
| PlayStation 3 system software | Sony Interactive Entertainment | 2006 | — |
| PlayStation 4 system software | Sony Interactive Entertainment | — | — |
| PlayStation 5 system software | Sony Interactive Entertainment | — | — |
| PlayStation Portable system software | — | — | — |
| PlayStation Portal system software | Sony Interactive Entertainment | — | — |
| PlayStation Vita system software | Sony Interactive Entertainment | — | — |
| Plurix | Federal University of Rio de Janeiro | — | — |
| Pocket PC 2002 | Microsoft | — | — |
| PocketPC 2000 | Microsoft | 2000 | — |
| Polippix | — | — | — |
| Pop!_OS | System76 | 2017 | GNU General Public License |
| Poseidon Linux | — | — | — |
| PowerNex | — | — | Mozilla Public License, version 2.0 |
| Prex | — | — | BSD licenses |
| PRIMOS | Prime Computer | — | — |