| Microsoft Windows 98 Second Edition | Microsoft | — | — |
| MIDAS | Microsoft | — | — |
| MIDIbox Operating System | — | — | — |
| MidnightBSD | — | 2007 | BSD licenses |
| Midori | Microsoft | — | — |
| MikeOS | — | — | BSD licenses |
| Mikros | — | — | — |
| MilaX | — | — | Common Development and Distribution License |
| MineHarmony OS | Huawei, CHN Energy | — | — |
| Minimal Linux Live | — | — | — |
| MiniOS | — | — | — |
| MINIX | Andrew S. Tanenbaum | — | 3-clause BSD License |
| MINIX 3 | Andrew S. Tanenbaum | — | BSD licenses |
| Minoca OS | — | — | GNU General Public License, version 3.0 |
| MiNT | — | — | — |
| MintPPC | — | — | — |
| Miosix | Federico Terraneo | 2009 | GNU General Public License |
| MIPS RISC/os | MIPS Technologies | — | — |
| Miracle Linux | Cybertrust | — | — |
| Miraculix | — | — | freeware |
| MirageOS | Thomas Gazagnaire | — | ISC license |
| MirOS BSD | — | — | BSD licenses |
| Miru directory server | — | — | — |
| MIUI | Xiaomi | — | GNU General Public License, version 3.0, Apache Software License 2.0 |
| MkLinux | Free software movement, Open Software Foundation | — | GNU General Public License |
| MNOS | — | — | — |
| Mobian | — | — | — |
| Mobilinux | MontaVista | 2005 | — |
| Moblin | Linux Foundation | — | — |
| MocaccinoOS | — | — | — |
| MoKee ROM | — | 2012 | MIT License, GNU General Public License, version 2.0 |
| Molinux | — | — | GNU General Public License |
| Momonga Linux | — | — | — |
| MonaOS | — | — | MIT License |
| MONECS | Monash University | — | — |
| Monoppix | — | — | — |
| MontaVista CGX Linux | MontaVista | — | — |
| Morphix | — | — | GNU General Public License |
| MorphOS | — | 2000 | — |
| MOS | — | — | — |
| Motoblur | Motorola Mobility | — | — |
| MotoMagx | Motorola | — | — |
| MoviX | — | — | GNU General Public License |
| 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 |