| AntiX | — | — | GNU General Public License |
| AO-DOS | — | — | — |
| AOKP | George Merlocco | — | Apache License |
| AOS/VS II | Data General | — | — |
| AOSC OS | Anthon Open Source Community | — | — |
| AospExtended | — | 2017 | — |
| Apache Mynewt OS | Apache Software Foundation | — | — |
| Apple CP/M | Apple Inc. | — | — |
| Apple DOS | Apple Inc. | 1978 | — |
| Apple GS/OS | Apple Inc. | — | — |
| Apple Pascal | Apple Inc. | — | — |
| Apple ProDOS | Apple Inc. | — | — |
| Apple SOS | Apple Inc. | — | — |
| Apricity OS | Alex Gajewski | — | — |
| Aptosid | — | 2006 | GNU General Public License |
| ArcaOS | Arca Noae | — | end-user license agreement |
| Arch BSD | — | — | — |
| Arch Hurd | — | 2011 | GNU General Public License |
| Arch Linux | Allan McRae, Anatol Pomozov, Aaron Griffin, Felix Yan | 2002 | GNU General Public License |
| Arch Linux 32 | — | — | — |
| Arch Linux ARM | — | — | — |
| ArchBang | — | 2010 | GNU General Public License |
| Archcraft | — | — | — |
| Archie Linux | — | — | — |
| ArchLabs | — | — | — |
| ArchOne | — | — | — |
| ArcoLinux | — | — | — |
| Ark Linux | — | 2005 | — |
| ArkOS | — | — | MIT License |
| ArkOS | — | — | GNU General Public License |
| ARMA aka Omoikane GNU/Linux | — | — | — |
| Armbian Linux | — | — | GNU General Public License, version 3.0 |
| Armed Forces mobile system | — | — | proprietary license |
| AROS Research Operating System | — | — | AROS Public License |
| Arthur operating system | Acorn Computers | — | — |
| ArtistX | — | — | — |
| Artix Linux | — | 2017 | — |
| ARX | Acorn Computers | — | — |
| Asahi Linux | Asahi Lina, Alyssa Rosenzweig, Hector Martin | 2021 | — |
| Asianux | — | — | — |
| AskoziaPBX | — | — | — |
| ASLinux Desktop | — | — | — |
| ASRI Edu | — | — | GNU Lesser General Public License, GNU General Public License |
| AsteroidOS | — | — | — |
| Astra Linux | RusBITech | — | GNU General Public License |
| AstraOS | — | — | — |
| Asturix | — | — | GNU General Public License |
| ASUS Express Gate | ASUS | — | — |