| Xbox system software | Microsoft | — | — |
| Xebian | — | — | — |
| Xerox Operating System | CII | — | — |
| Xinu | Douglas Comer | — | — |
| XMK | — | — | — |
| XOberon | ETH Zurich | — | — |
| XOS | — | — | — |
| Xous | Betrusted | — | Apache Software License 2.0 |
| XTS-400 | BAE Systems | — | — |
| Xv6 | Massachusetts Institute of Technology | — | MIT License |
| Yandex.Kit | — | — | proprietary license |
| Z/VM | IBM | — | — |
| Z9001-OS | VEB Robotron-Meßelektronik „Otto Schön“ Dresden | — | — |
| Zapple Monitor | Princeton | — | — |
| ZDOS | — | — | — |
| Zeroshell | — | — | GNU General Public License |
| Zeta | YellowTAB | — | — |
| Zex/OS | — | — | — |
| ZFSGuru | — | — | — |
| ZRouter | — | — | — |
| ZXVGS | — | — | — |
| Ładowacz | — | — | — |
| ΜnOS | — | — | — |