| SharpOS | — | — | GNU Lesser General Public License, BSD licenses, GNU General Public License, version 2.0 |
| Shrine | — | — | Unlicense |
| SIBO | Psion | — | — |
| 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 |
| Slackware | Patrick Volkerding | — | GNU General Public License |
| SlapOS | — | — | — |
| SLES15 SP2 | — | — | — |
| SlugOS | — | — | — |
| Smarterphone | — | — | — |
| Smartisan OS | — | — | — |
| SMSQ/E | Tony Tebby | — | — |
| Snap OS | Snap Inc. | — | — |
| SO | Institute of Mathematical Machines | — | — |
| Soft Hard Real-Time Kernel | Sant'Anna School of Advanced Studies | — | GNU General Public 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 | — |
| SONiC | Microsoft, Open Compute Project | 2016 | Apache License, GNU General Public License |
| Sophos Firewall OS | — | — | — |
| SOX Unix | BB Tecnologia e Serviços | — | proprietary license |
| SparkyLinux | — | — | GNU General Public License |
| SPIN | University of Washington | — | — |
| Splashtop OS | Splashtop Inc. | — | — |
| Spring | Sun Microsystems | — | — |
| StartCom Linux | — | — | — |
| StartOS | — | — | — |
| Stratus VOS | Stratus Technologies | — | — |
| Subgraph OS | — | — | GNU General Public License |
| SunOS | Sun Microsystems | — | proprietary license |
| Sunwah Linux | — | — | — |
| SUPER-UX | NEC | — | — |
| SuperGamer | — | — | — |
| Svarog386 | — | — | — |
| Sx-window | Sharp Corporation, Hudson Soft | 1989 | — |