| SymbOS | — | — | — |
| Symobi | Miray Software | — | — |
| 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 | — | — | — |
| T-Kernel | — | — | — |
| TeleSec Chipcard Operating System | T-Systems International GmbH | — | — |
| TempleOS | Terry A. Davis | 2013 | — |
| TENEX | BBN Technologies | 1969 | — |
| THE multiprogramming system | Eindhoven University of Technology | — | — |
| Theseus | — | — | MIT License |
| Thinstation | — | — | GNU General Public License |
| Thoth (operating system) | University of Waterloo | — | — |
| Time Sharing Operating System | RCA Corporation | 1968 | — |
| Time Warp Operating System | Jet Propulsion Laboratory | — | — |
| Tin Hat Linux | — | — | — |
| TinyKRNL | — | — | — |
| TinyOS | — | — | BSD licenses |
| TI‐RTOS | Texas Instruments | — | BSD licenses |
| TOPS-10 | Digital Equipment Corporation | — | — |
| TOPS-20 | Leonard Bosack, Daniel Murphy, Digital Equipment Corporation | 1976 | — |
| Tor-ramdisk | — | — | GNU General Public License, version 3.0 |
| Towns OS | — | — | — |
| TR-DOS | — | — | — |
| Transaction Management eXecutive | NCR Corporation | — | — |
| Transaction Processing Facility | IBM | 1979 | — |
| Tri-Lab Operating System | — | — | — |
| TRIPOS | MetaComCo, University of Cambridge | 1978 | — |
| Trix | MIT Computer Science and Artificial Intelligence Laboratory | — | — |
| TRSDOS | Tandy Corporation | — | — |
| TrueNAS | IXsystems | — | proprietary license, GNU General Public License, version 3.0, BSD licenses |
| TSS-8 | Digital Equipment Corporation | 1968 | proprietary license |
| TSS/360 | IBM | 1967 | — |
| TSX-32 | — | — | — |
| TSX-Plus | — | — | — |
| Tukaani Linux | — | — | — |
| TurboDOS | — | — | — |
| Turtlebot | — | — | BSD licenses |
| TvOS 10 | Apple Inc. | — | Apple Public Source License |
| TvOS 11 | — | — | — |
| TvOS 12 | — | — | — |
| TvOS 13 | Apple Inc. | 2019 | Apple Public Source License |
| TvOS 14 | Apple Inc. | 2020 | Apple Public Source License |