| MS-DOS 4.0 | Microsoft | 1988 | MIT License |
| MS-DOS 6.22 | Microsoft | 1994 | proprietary license |
| MS-DOS 7 | Microsoft | — | proprietary license |
| MS/8 | Richard Lary | — | — |
| MSX-DOS | Microsoft | — | — |
| Mubuntu | — | — | — |
| MuLinux | — | — | GNU General Public License |
| Multi-Environment Real-Time | Bell Labs | — | — |
| Multi-tasking MS-DOS 4.0 | Microsoft | 1986 | MIT License |
| Multics | Bell Labs, Massachusetts Institute of Technology, General Electric | 1969 | MIT License |
| Multiple Console Time Sharing System | General Motors Research Laboratories | — | — |
| Multiuser DOS | Digital Research | — | — |
| MULTOS | — | — | — |
| MUSIC/SP | McGill University | 1972 | — |
| Musix GNU/Linux | — | — | — |
| MVS | IBM | 1974 | proprietary license |
| MX Linux | — | — | GNU General Public License |
| MyLinux | — | — | — |
| Mythbuntu | — | — | GNU General Public License |
| MythDora | — | — | — |
| Möbius | — | — | GNU General Public License |
| NanoQplus | Electronics and Telecommunications Research Institute | — | — |
| NASLite | — | — | — |
| Navigatrix | — | — | GNU General Public License |
| NCOS | Acorn Computers, Pace plc | — | — |
| NCR VRX | NCR Corporation | — | — |
| Nemesis | University of Cambridge | — | — |
| NeoKylin | — | — | proprietary license |
| NepaLinux | Madan Puraskar Pustakalaya | — | GNU General Public License |
| NetBSD | — | 1993 | BSD licenses |
| NetHunter | — | — | — |
| Netrunner | Blue Systems | — | GNU General Public License |
| NetSecL OS | — | — | GNU General Public License |
| NetWare | Novell | — | — |
| NewDos/80 | — | — | — |
| NewOS | Travis Geiselbrecht | — | — |
| Newton OS | Apple Inc. | — | — |
| Newtoos | — | — | — |
| Nexenta OS | Nexenta Systems | — | Common Development and Distribution License version 1.0 |
| NexentaStor | Nexenta Systems | — | — |
| NeXT Computer | — | 1988 | — |
| NextBSD | — | — | BSD licenses |
| NeXTSTEP | NeXT | — | proprietary license |
| NimbleX | — | — | — |
| Nintendo 3DS system software | Nintendo | — | — |
| Nintendo DSi system software | Nintendo | — | — |
| Nintendo IOS | Nintendo | — | — |
| Nintendo Switch system software | HAL Laboratory, Nintendo | 2017 | proprietary license |