| PrivatOS | Silent Circle | 2014 | — |
| Proxmox Backup Server | — | — | GNU Affero General Public License, version 3.0 |
| Proxmox Virtual Environment | Proxmox Server Solutions GmbH | — | GNU Affero General Public License, version 3.0 |
| PTS-DOS | Paragon Software Group | 1993 | proprietary license |
| Puredyne | — | — | — |
| PWB/UNIX | Bell Labs | 1977 | — |
| QNodeOS | QuTech | — | — |
| QNX | BlackBerry, QNX Software Systems | — | — |
| QP | — | — | GNU General Public License |
| Qt Extended | Qt Group Plc | — | GNU General Public License |
| R9 | — | — | MIT License |
| RadiOS | — | — | — |
| Rainbow-OS | Ulm University | — | — |
| RAMON | — | — | — |
| RDOS | Data General | — | — |
| Redox | Jeremy Soller | — | MIT License |
| RedSleeve | — | — | GNU General Public License |
| Research Unix | Bell Labs, Ken Thompson, Dennis M. Ritchie, Brian Kernighan | — | — |
| REX OS | Qualcomm | — | — |
| Rhapsody | Apple Inc. | — | — |
| RISC iX | Acorn Computers | — | — |
| RISC OS | Acorn Computers, RISCOS Ltd, RISC OS Open, Castle Technology | — | freeware |
| RMX | TenAsys, Intel | — | — |
| Robigalia | — | — | — |
| ROCK Linux | — | — | GNU General Public License |
| Rockbox | — | 2002 | GNU General Public License, version 2.0 |
| Rodos | German Aerospace Center | — | BSD licenses |
| ROM-DOS | Datalight | — | — |
| RPath | — | — | — |
| RSTS/E | Digital Equipment Corporation | — | — |
| RSX-11 | Digital Equipment Corporation | — | — |
| RT-11 | Digital Equipment Corporation | — | proprietary license |
| RTMOS | General Electric | — | — |
| RTX | IntervalZero | — | — |
| RTXC Quadros | — | — | — |
| RunnerOn | — | — | — |
| Russian Fedora Remix | — | — | — |
| S5-DOS/MT | Siemens | — | — |
| SabineOS | — | — | — |
| Sacix | — | — | GNU General Public License |
| Samsung Experience | Samsung Group | — | — |
| Scout | University of Arizona | — | — |
| ScreenOS | Juniper Networks | — | — |
| Semplice Linux | — | — | — |
| SerenityOS | Andreas Kling | — | 2-clause BSD License |
| Series 30+ | — | 2013 | — |
| Series 40 | — | — | — |
| SHARE Operating System | SHARE | — | — |