| MP/M | Digital Research, Tom Rolander | — | proprietary license |
| MQX | — | — | — |
| MS-DOS | Microsoft | 1981 | MIT License, proprietary license |
| MS-DOS 1.25 | Microsoft | 1982 | MIT License |
| MS-DOS 2.0 | Microsoft | 1983 | MIT License |
| 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 | — | — |
| 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 | — |
| MVS | IBM | 1974 | proprietary license |
| Möbius | — | — | GNU General Public License |
| NASLite | — | — | — |
| NCOS | Acorn Computers, Pace plc | — | — |
| NCR VRX | NCR Corporation | — | — |
| Nemesis | University of Cambridge | — | — |
| NeoKylin | — | — | proprietary license |
| NetBSD | — | 1993 | BSD licenses |
| NetWare | Novell | — | — |
| NewDos/80 | — | — | — |
| NewOS | Travis Geiselbrecht | — | — |
| Newton OS | Apple Inc. | — | — |
| NextBSD | — | — | BSD licenses |
| Nintendo 3DS system software | Nintendo | — | — |
| Nintendo DSi system software | Nintendo | — | — |
| Nintendo IOS | Nintendo | — | — |
| Nitix | — | — | — |
| NLTSS | Lawrence Livermore National Laboratory | — | — |
| Nokia Fastlane | Nokia | — | — |
| Nokia X | — | — | — |
| NomadBSD | — | 2018 | 2-clause BSD License |
| NonStop OS | Tandem Computers | — | — |
| NORD | — | — | — |
| NorthStar DOS | North Star Computers | — | — |
| NORTON-BK | — | — | — |
| NOS | Alien Internet Services | — | — |
| NOS | Control Data Corporation | — | — |
| NOS/VE | Control Data Corporation | — | — |
| Not Another Completely Heuristic Operating System | — | — | BSD licenses |
| Nothing OS | Nothing Technology Ltd. | — | Android Open Source Project |
| Novell S-Net | Novell | — | — |