| Sintran | — | 1968 | proprietary license |
| Sintran III | Norsk Data | 1974 | — |
| Siris 8 | CII | — | — |
| SISNE plus | Itautec, Scopus Tecnologia | — | — |
| SkiftOS | — | — | MIT License |
| SkyOS | Robert Szeleney | 1997 | — |
| Slackware | Patrick Volkerding | — | GNU General Public License |
| Slamd64 | — | — | — |
| SlapOS | — | — | — |
| Slax | — | — | GNU General Public License |
| SLES15 SP2 | — | — | — |
| SlimPup | — | — | — |
| SlimRom | — | — | — |
| SlimRoms | — | — | — |
| Slis | — | — | — |
| SliTaz | — | — | GNU General Public License |
| SlugOS | — | — | — |
| Smarterphone | — | — | — |
| Smartisan OS | — | — | — |
| SmartOS | Joyent | — | Common Development and Distribution License version 1.0 |
| SME Server | — | — | GNU General Public License |
| SmoothWall | — | — | GNU General Public License, version 2.0 |
| SMSQ/E | Tony Tebby | — | — |
| Snap OS | Snap Inc. | — | — |
| Snøfrix | — | — | — |
| SO | Institute of Mathematical Machines | — | — |
| Soft Hard Real-Time Kernel | Sant'Anna School of Advanced Studies | — | GNU General Public License |
| Softlanding Linux System | Peter MacDonald | 1992 | GNU General Public License |
| Solaris | Sun Microsystems, Oracle Corporation | 1991 | proprietary license |
| Solaris version 1.x | — | 1991 | — |
| Solaris version 10 | — | — | — |
| Solaris version 11 Express 2010.11 | — | — | — |
| Solaris version 11.1 | — | — | — |
| Solaris version 11.2 | — | — | — |
| Solaris version 11.3 | — | 2015 | — |
| Solaris version 2.0 | — | 1992 | — |
| Solaris version 7 update 3/99 | — | 1999 | — |
| Solaris version 8 update 6/00 | — | 2000 | — |
| Solaris version 9 | — | 2002 | — |
| Solus | — | — | GNU General Public License |
| SolydXK | — | 2013 | — |
| SONiC | Microsoft, Open Compute Project | 2016 | Apache License, GNU General Public License |
| Sophos Firewall OS | — | — | — |
| Sorcerer | — | — | — |
| Sortix | Jonas Termansen | 2011 | ISC license |
| Soso | — | — | 2-clause BSD License |
| Source Mage | — | — | — |
| SOX Unix | BB Tecnologia e Serviços | — | proprietary license |