| /e/OS | Gaël Duval, E Foundation | 2018 | MIT License, Apache Software License 2.0 |
| 386BSD | William Jolitz, Lynne Jolitz | 1992 | BSD licenses |
| 3CX Phone System | — | — | end-user license agreement |
| 4K Disk Monitor System | Digital Equipment Corporation | — | — |
| 4MLinux | — | — | — |
| 64 Studio | — | — | GNU General Public License |
| 68K/OS | GST Computer Systems | — | — |
| 86-DOS | Seattle Computer Products, Tim Paterson | 1980 | — |
| 9front | — | — | MIT License |
| A/ROSE | Apple Inc. | — | — |
| A/UX | Apple Inc. | — | proprietary license |
| A2 | ETH Zurich, Niklaus Wirth | — | BSD licenses |
| ABC GNU/Linux | — | — | — |
| ABCbuntu | — | — | — |
| Abitti | The Matriculation Examination Board of Finland | — | — |
| Absolute Linux | — | — | GNU General Public License |
| AbulÉdu Live | — | — | GNU General Public License |
| Acorn MOS | Acorn Computers | — | — |
| ACOS-4 | NEC | — | — |
| ACOS-6 | NEC | — | — |
| Adamantix | — | — | GNU General Public License |
| AdiOS | AdiOS | — | proprietary license |
| AdJ | — | — | — |
| Advanced Comprehensive Operating System | NEC | — | — |
| Advanced XPerience OS | — | — | — |
| Adélie Linux | — | — | — |
| AerOS | — | — | — |
| AerynOS | — | — | — |
| Ageless Linux | — | — | — |
| AgiliaLinux | — | — | — |
| AICP | — | — | — |
| Akaros | — | — | GNU General Public License, version 2.0 |
| Alinex | — | — | — |
| ALinux | — | — | — |
| AliOS | Alibaba Group | — | — |
| AlliedWare Plus | Allied Telesis Holdings | — | — |
| AlmaLinux | Andrew Lukoshko | — | — |
| AlphaMicro Operating System | Alpha Microsystems | — | — |
| Alpine Linux | — | — | 2-clause BSD License, GNU General Public License, version 2.0, MIT License |
| ALT Linux | ALT Linux Team, ALT Linux | — | GNU General Public License, version 2.0 or later |
| Alter Linux | — | — | — |
| Altimit OS | — | — | — |
| AmayaOS | — | 2014 | GNU General Public License, version 3.0 |
| Amazon Linux | Amazon | 2011 | — |
| Amber Linux | — | — | — |
| AMIBIOS | American Megatrends | 1985 | proprietary license |
| Amiga Unix | Commodore International | — | — |
| AmigaDOS | Commodore International | — | proprietary license |