| 4K Disk Monitor System | Digital Equipment Corporation | — | — |
| 68K/OS | GST Computer Systems | — | — |
| 86-DOS | Seattle Computer Products, Tim Paterson | 1980 | — |
| A/ROSE | Apple Inc. | — | — |
| A/UX | Apple Inc. | — | proprietary license |
| A2 | ETH Zurich, Niklaus Wirth | — | BSD licenses |
| ABC GNU/Linux | — | — | — |
| Abitti | The Matriculation Examination Board of Finland | — | — |
| Acorn MOS | Acorn Computers | — | — |
| ACOS-4 | NEC | — | — |
| ACOS-6 | NEC | — | — |
| AdiOS | AdiOS | — | proprietary license |
| AdJ | — | — | — |
| Advanced Comprehensive Operating System | NEC | — | — |
| Advanced XPerience OS | — | — | — |
| AerynOS | — | — | — |
| Ageless Linux | — | — | — |
| Akaros | — | — | GNU General Public License, version 2.0 |
| Alinex | — | — | — |
| AlliedWare Plus | Allied Telesis Holdings | — | — |
| AlphaMicro Operating System | Alpha Microsystems | — | — |
| Altimit OS | — | — | — |
| Amazon Linux | Amazon | 2011 | — |
| Amiga Unix | Commodore International | — | — |
| AmigaOS | Hyperion Entertainment, Haage & Partner, Commodore International | — | end-user license agreement |
| AmigaOS 4 | Hyperion Entertainment | 2006 | proprietary license |
| AmiKit | — | — | proprietary license |
| AMSDOS | Amstrad | — | — |
| Analogue OS | Analogue Inc. | — | — |
| ANDOS | — | — | — |
| Android 1.0 | — | — | Apache Software License 2.0 |
| Android 1.1 | — | 2009 | — |
| Android 10 | Google, Open Handset Alliance | 2019 | Apache License |
| Android 11 | Open Handset Alliance | — | — |
| Android Auto | Google | 2015 | proprietary license |
| Android Automotive | — | 2017 | — |
| Android Cupcake | Google | — | — |
| Android Donut | Google | — | — |
| Android Eclair | Google | — | — |
| Android Froyo | Google | — | — |
| Android Gingerbread | Google | 2010 | — |
| Android Honeycomb | Google | — | — |
| Android Ice Cream Sandwich | Google | — | — |
| Android KitKat | Open Handset Alliance, Google | — | Apache License, GNU General Public License |
| Android Lollipop | — | 2014 | Apache Software License 2.0, Android Open Source Project, GNU General Public License, version 2.0 |
| Android Marshmallow | Open Handset Alliance, Google | — | free software, Android Open Source Project |
| Android Nougat | Google, Open Handset Alliance | 2016 | Android Open Source Project |
| Android Open Source Project | — | — | Creative Commons Attribution 2.5 Generic, GNU General Public License, Apache License |