| 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 AIX | IBM | 1986 | proprietary license |
| IBM Basic Programming Support | IBM | — | — |
| IBM CP-40 | Cambridge Scientific Center | — | — |
| IBM i | IBM | — | — |
| IBM PC DOS | IBM | 1981 | proprietary license |
| IBM System/34, 36 System Support Product | IBM | — | — |
| IBM TopView | IBM | — | proprietary license |
| IBM Workplace OS | IBM | — | — |
| IBuntu | — | — | — |
| Icaros Desktop | — | — | — |
| ICL VME | International Computers Limited | — | — |
| IDEDOS | — | 1996 | GNU General Public License, version 2.0 |
| Idris | P. J. Plauger | — | commercial software, proprietary license |
| Iglunix | — | — | Zero-clause BSD License |
| IGolaware Linux | — | — | — |
| Illumos | — | — | Common Development and Distribution License version 1.0 |
| IMagic OS | — | — | — |
| Imagineos | — | — | — |
| IMAX 432 | Intel | — | — |
| IMDOS | IMS Associates | — | — |
| IMOS | NCR Corporation | — | — |
| Impi Linux | — | — | — |
| Incognito | — | 2008 | GNU General Public License |
| Incompatible Timesharing System | Massachusetts Institute of Technology | — | GNU General Public License v3.0 or later |
| IndLinux | — | — | GNU Lesser General Public License, version 2.1 or later, GNU General Public License, version 2.0 |
| Inferno | Vita Nuova Holdings, Bell Labs | — | GNU Lesser General Public License, MIT License, GNU General Public License |
| InfinitiveOS | — | — | — |
| InfraLinux | — | — | — |
| Injector Linux | — | — | GNU General Public License |
| INK | — | — | — |
| INMOS | Institute of Electronic Control Machines | — | — |
| Inquisitor | — | — | GNU General Public License |
| Inspur K-UX | Inspur | — | — |
| Instant WebKiosk | — | — | — |
| Integrity | Green Hills Software | — | — |
| INtime | — | — | — |
| IodéOS | — | — | Apache Software License 2.0, GNU Affero General Public License, version 3.0 |
| IOS | Apple Inc. | 2007 | proprietary license |
| IOS 10 | Apple Inc. | 2016 | end-user license agreement |
| IOS 11 | Apple Inc. | — | end-user license agreement |
| IOS 12 | Apple Inc. | — | — |
| IOS 13 | Apple Inc. | 2019 | proprietary license |
| IOS 14 | Apple Inc. | 2020 | — |
| IOS 15 | Apple Inc. | — | — |
| IOS 16 | Apple Inc. | 2022 | — |
| IOS 17 | Apple Inc. | 2023 | — |
| IOS 18 | Apple Inc. | 2024 | — |
| IOS 26 | Apple Inc. | 2025 | — |
| IOS 27 | Apple Inc. | — | — |
| IOS 4 | Apple Inc. | 2010 | Apple Public Source License |
| IOS 5 | Apple Inc. | — | Apple Public Source License |
| IOS 6 | Apple Inc. | 2012 | Apple Public Source License |
| IOS 7 | Apple Inc. | 2013 | Apple Public Source License |
| IOS 8 | Apple Inc. | 2014 | Apple Public Source License |
| IOS 9 | Apple Inc. | 2015 | end-user license agreement |
| IPadOS | Apple Inc. | 2019 | proprietary license |
| IPadOS 14 | Apple Inc. | — | — |
| IPadOS 15 | Apple Inc. | — | — |
| IPadOS 16 | Apple Inc. | 2022 | — |
| IPadOS 17 | Apple Inc. | 2023 | — |
| IPadOS 18 | Apple Inc. | — | — |
| IPadOS 26 | — | — | — |
| IPadOS 27 | Apple Inc. | — | — |
| IPCop | — | 2001 | GNU General Public License |
| IPFire | — | — | GNU General Public License |
| IPhone Linux | — | 2008 | — |
| IPhone OS 1 | Apple Inc. | 2007 | proprietary license |
| IPhone OS 2 | Apple Inc. | 2008 | Apple Public Source License |
| IPhone OS 3 | Apple Inc. | 2009 | Apple Public Source License |
| IPodLinux | — | — | GNU General Public License, version 2.0 |
| IRIX | Silicon Graphics | 1988 | — |
| IS-DOS | — | — | — |
| ISIS | Intel | — | — |
| Iskolinux | — | — | — |
| Issabel | — | — | GNU General Public License |
| ISURU Linux | — | — | GNU General Public License |
| ITIX | — | — | GNU General Public License |
| ITsART | — | — | — |