| Xbox One and Xbox Series X/S system software | Microsoft | — | — |
| Xbox system software | Microsoft | — | — |
| XCP-ng | — | — | GNU General Public License, version 2.0 or later |
| Xebian | — | — | — |
| Xenix | Microsoft, Santa Cruz Operation, AT&T Corporation | — | proprietary license |
| Xenon Linux | — | — | — |
| Xenta OS | — | — | — |
| Xero Linux | — | — | — |
| Xerox Operating System | CII | — | — |
| Xevuan | — | — | — |
| Xiaomi HyperOS | Xiaomi | 2023 | — |
| Xiaomi.eu | — | — | — |
| Xinu | Douglas Comer | — | — |
| Xinuos OpenServer | Santa Cruz Operation, Xinuos | — | proprietary license |
| Xinux | UZINFOCOM | — | GNU General Public License |
| XMK | — | — | — |
| XOberon | ETH Zurich | — | — |
| XOS | — | — | — |
| Xous | Betrusted | — | Apache Software License 2.0 |
| XTOS | Tensilica | — | — |
| XTS-400 | BAE Systems | — | — |
| Xubuntu | Canonical Ltd., Ubuntu Foundation | 2006 | — |
| Xv6 | Massachusetts Institute of Technology | — | MIT License |
| Yandex.Kit | — | — | proprietary license |
| Yanux | Luch Scientific Production Association | — | — |
| Yellow Dog Linux | Fixstars Solutions | — | GNU General Public License |
| Yggdrasil Linux/GNU/X | Yggdrasil Computing | — | — |
| Yocto Project | — | 2011 | — |
| YOPER | — | — | — |
| YOUrOS | — | — | — |
| YunoHost | — | — | GNU Affero General Public License, version 3.0 |
| Z/OS | IBM | 2001 | — |
| Z/VM | IBM | — | — |
| Z9001-OS | VEB Robotron-Meßelektronik „Otto Schön“ Dresden | — | — |
| Zapple Monitor | Princeton | — | — |
| ZDOS | — | — | — |
| Zentyal | — | — | GNU General Public License |
| Zenwalk | — | — | GNU General Public License |
| Zephix | — | — | — |
| Zephyr | Wind River Systems, Linux Foundation | — | Apache Software License 2.0 |
| Zeroshell | — | — | GNU General Public License |
| Zeta | YellowTAB | — | — |
| ZevenOS | — | — | GNU General Public License |
| Zex/OS | — | — | — |
| ZFSGuru | — | — | — |
| Zorin OS | Zorin Technology Group Ltd. | 2009 | GNU General Public License |
| ZRouter | — | — | — |
| ZXVGS | — | — | — |