| S5-DOS/MT | Siemens | — | — |
| S60 | Nokia | — | — |
| Sabayon Linux | — | — | GNU General Public License |
| Sabily | — | — | GNU General Public License |
| SabineOS | — | — | — |
| Sacix | — | — | GNU General Public License |
| Sailfish OS | Jolla | — | — |
| Salix OS | — | — | GNU General Public License, version 3.0, GNU General Public License, version 2.0, Creative Commons Attribution-ShareAlike 3.0 Unported, BSD licenses |
| SambaEdu | — | — | GNU General Public License |
| SAMDOS | Miles Gordon Technology | — | — |
| SAMity Linux | — | — | — |
| Samsung Experience | Samsung Group | — | — |
| Satux | — | — | — |
| SchilliX | — | — | Common Development and Distribution License version 1.0, GNU General Public License |
| Scientific Linux | Fermilab | — | GNU General Public License |
| SCore | RWCP | — | — |
| Scout | University of Arizona | — | — |
| ScreenOS | Juniper Networks | — | — |
| SeaBIOS | — | — | GNU Lesser General Public License, version 3.0 |
| Secureblue | — | — | Apache Software License 2.0 |
| Security Onion | — | — | — |
| Seminarix | — | — | — |
| Semplice Linux | — | — | — |
| SEP OS | Apple Inc. | — | — |
| Serbian GNU/Linux | — | — | — |
| SerenityOS | Andreas Kling | — | 2-clause BSD License |
| Series 30 | — | — | — |
| Series 30+ | — | 2013 | — |
| Series 40 | — | — | — |
| SHARE Operating System | SHARE | — | — |
| SharpOS | — | — | GNU Lesser General Public License, BSD licenses, GNU General Public License, version 2.0 |
| Shinux | — | — | free software license |
| SHR | — | — | GNU General Public License |
| ShredOS | — | — | GNU General Public License, version 2.0 |
| Shrine | — | — | Unlicense |
| SIBO | Psion | — | — |
| Siduction | — | 2011 | GNU General Public License |
| Sigma File Manager | — | — | GNU General Public License v3.0 or later |
| SimpleLinux | — | — | — |
| Sinclair QDOS | Sinclair Research | 1984 | proprietary license |
| Single User Control Program | VEB Robotron | — | — |
| SINIX | Fujitsu Siemens Computers, Siemens Nixdorf Informationssysteme | — | — |
| 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 |
| SPARC64/OS | HAL Computer Systems | — | — |
| SparkyLinux | — | — | GNU General Public License |
| SPBLinux | — | — | GNU General Public License |
| Special Real Time Operating System | IBM | — | — |
| SPIN | University of Washington | — | — |
| SpiralLinux | — | — | — |
| SPIX | Groupe Bull | — | proprietary license |
| Splashtop | — | — | proprietary license |
| Splashtop OS | Splashtop Inc. | — | — |
| Spring | Sun Microsystems | — | — |
| Sprite | University of California, Berkeley | — | MIT License |
| Stali | Anselm R. Garbe | — | — |
| StartCom Linux | — | — | — |
| StartOS | — | — | — |
| StatiXOS | — | — | — |
| SteamOS | Valve Corporation | — | — |
| Stratus VOS | Stratus Technologies | — | — |
| Subgraph OS | — | — | GNU General Public License |
| Suicide Linux | — | 2009 | proprietary license |
| SunOS | Sun Microsystems | — | proprietary license |
| Sunwah Linux | — | — | — |
| SUPER-UX | NEC | — | — |
| SuperGamer | — | — | — |
| SuperX | — | — | — |
| SuSE Linux | SUSE, Novell | — | GNU General Public License |
| SUSE Linux Enterprise Desktop | SUSE | — | GNU Lesser General Public License |
| SUSE Linux Enterprise Server | SUSE | — | GNU General Public License |
| Svarog386 | — | — | — |
| Sx-window | Sharp Corporation, Hudson Soft | 1989 | — |
| Syllable Desktop | — | — | GNU General Public License |
| Symbian | Symbian Foundation, Nokia, Symbian Software | — | proprietary license |
| SymbOS | — | — | — |
| Symobi | Miray Software | — | — |
| Symphony OS | — | — | GNU General Public License |
| SymplyOS | — | — | — |
| System 1 | Apple Inc. | 1984 | proprietary license |
| System 2 | Apple Inc. | — | — |
| System 3 | Apple Inc. | — | — |
| System 4 | Apple Inc. | — | — |
| System 6 | Apple Inc. | — | proprietary license |
| System 7 | Apple Inc. | — | proprietary license |
| System Software 5 | — | — | — |
| SystemRescue | — | — | GNU General Public License |