| 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 | — |
| 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 | — | — |
| Pocket PC 2002 | Microsoft | — | — |
| PocketPC 2000 | Microsoft | 2000 | — |
| Polippix | — | — | — |
| Pop!_OS | System76 | 2017 | GNU General Public License |
| Porteus | — | — | GNU General Public License |
| Poseidon Linux | — | — | — |
| PowerLinux | IBM | — | — |
| PowerNex | — | — | Mozilla Public License, version 2.0 |
| Predator OS | — | — | — |
| Prex | — | — | BSD licenses |
| PRIMOS | Prime Computer | — | — |
| PrimTux | — | — | GNU General Public License, version 3.0 |
| PrivatOS | Silent Circle | 2014 | — |
| Progeny Componentized Linux | Progeny Linux Systems | — | — |
| ProteanOS | — | — | — |
| Proxmox Backup Server | — | — | GNU Affero General Public License, version 3.0 |
| Proxmox Mail Gateway | — | — | — |
| Proxmox Virtual Environment | Proxmox Server Solutions GmbH | — | GNU Affero General Public License, version 3.0 |
| PTS-DOS | Paragon Software Group | 1993 | proprietary license |
| Puavo | — | — | GNU General Public License, version 2.0 |
| Publik | — | — | GNU General Public License, Affero General Public License |
| PUD | — | — | — |
| Puppy Linux | — | 2003 | GNU General Public License |
| PureDarwin | — | — | — |
| Puredyne | — | — | — |
| PureNexusProject | — | 2012 | — |
| PureOS | Purism | — | Creative Commons Attribution-ShareAlike 4.0 International |
| PWB/UNIX | Bell Labs | 1977 | — |
| PX5 RTOS | William Lamie | — | — |
| Pyabr | — | — | — |