| Gentoo/FreeBSD | — | — | — |
| GEORGE | International Computers and Tabulators | — | — |
| GEOS | GeoWorks | 1990 | Apache License |
| Ghost | — | — | GNU General Public License, version 3.0 |
| GhostBSD | — | 2009 | GNU General Public License, 2-clause BSD License |
| Glass OS | Google | — | — |
| GlobalView | Xerox | 1986 | — |
| GM-NAA I/O | General Motors Research Laboratories, North American Aviation | — | — |
| GMOS | General Motors | — | — |
| GNO/ME | — | — | — |
| GNOSIS | Tymshare | — | — |
| GNU | GNU Project | — | GNU Lesser General Public License, GNU General Public License, GNU Affero General Public License |
| GNU/Hurd | — | — | — |
| GNU/Linux | Richard Stallman, Linus Torvalds | — | GNU General Public License |
| Goobuntu | — | — | — |
| Googlebook OS | Google DeepMind, Google | — | — |
| GP32 System Software | — | — | — |
| Greenbone OS | Greenbone AG | — | — |
| Group Control System | IBM | — | — |
| Haiku | Michael Phipps, Haiku Inc. | 2002 | MIT License |
| Halium | — | — | — |
| HardenedBSD | — | — | — |
| Harmony (operating system) | — | — | — |
| HarmonyOS NEXT | Huawei | 2023 | — |
| HDOS | Heath Company, Gordon Letwin | — | — |
| HelenOS | — | — | 3-clause BSD License |
| HeliOS | Perihelion Software | — | — |
| HelloSystem | — | — | — |
| Hikarunix | — | — | — |
| Hitachi VOS | Hitachi | — | — |
| HomeOS | Apple Inc. | — | — |
| HomeOS | Microsoft | — | — |
| Honeywell CP-6 | Honeywell | — | — |
| House | — | — | BSD licenses |
| Hybryde Linux | — | — | — |
| HyperSpace | Phoenix Technologies | — | — |
| I5 OS | IBM | — | — |
| IBM 4680 OS | IBM | — | — |
| IBM 4690 OS | IBM, Toshiba | — | — |
| IBM 7090/94 IBSYS | IBM | 1960 | — |
| IBM 8100 DPCX | IBM | — | — |
| IBM 8100 DPPX | IBM | 1978 | — |
| IBM Academic Operating System | IBM | — | — |
| IBM Airline Control Program | IBM | 1968 | — |
| IBM Basic Programming Support | IBM | — | — |
| IBM CP-40 | Cambridge Scientific Center | — | — |
| IBM PC DOS | IBM | 1981 | proprietary license |
| IBM System/34, 36 System Support Product | IBM | — | — |