| PC/IX | Interactive Systems Corporation | — | — |
| PCLinuxOS | — | — | — |
| PCQ Linux 2005 | — | — | — |
| PDOS | — | — | — |
| Pear OS | — | — | — |
| PEARL Operating System | Brown, Boveri & Cie | — | — |
| PearOS | — | — | — |
| PelicanHPC | — | — | — |
| PenDOS | Communication Intelligence Corporation (United States) | — | — |
| Pentoo | — | — | GNU General Public License |
| Peppermint Linux OS | — | — | GNU General Public License |
| Persia OS | — | 2024 | — |
| PfSense | — | — | Apache License |
| Phantom OS | — | — | GNU Lesser General Public License, version 3.0 |
| PHLAK | — | — | — |
| Phoenix BIOS | Phoenix Technologies | — | — |
| Phoenix OS | — | — | — |
| Phoenix-RTOS | — | — | — |
| PhoeNUX OS | — | — | — |
| Photon OS | — | — | — |
| Pi-KVM | — | — | GNU General Public License, version 3.0 |
| Pick operating system | — | — | — |
| PICO OS | PICO | — | — |
| Pidora | Seneca Polytechnic | 2012 | — |
| Pie Box Enterprise Linux | — | — | — |
| PikeOS | SYSGO | — | — |
| Pilot | Palo Alto Research Center | — | — |
| PING | — | — | GNU General Public License |
| Pingo Linux | — | — | — |
| Pinguy OS | — | — | GNU General Public License |
| PiPlay | — | — | — |
| Piumalinux | — | — | — |
| Pixart | — | — | — |
| PixelExperience | — | 2019 | — |
| Plamo Linux | — | — | — |
| Plan 9 | Bell Labs | 1992 | GNU General Public License, version 2.0, MIT License, Lucent Public License |
| Plan B | King Juan Carlos University | — | — |
| Plasma Mobile | KDE | — | GNU Lesser General Public License, version 2.1, GNU General Public License, version 2.0, GNU Library General Public License, version 2.0 |
| Platypux | — | — | GNU General Public License |
| 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 | — | — |
| PLD Linux | — | 1998 | GNU General Public License |
| Plurix | Federal University of Rio de Janeiro | — | — |