| IBM TopView | IBM | — | proprietary license |
| IBM Workplace OS | IBM | — | — |
| Icaros Desktop | — | — | — |
| ICL VME | International Computers Limited | — | — |
| IDEDOS | — | 1996 | GNU General Public License, version 2.0 |
| Idris | P. J. Plauger | — | commercial software, proprietary license |
| IMAX 432 | Intel | — | — |
| IMDOS | IMS Associates | — | — |
| IMOS | NCR Corporation | — | — |
| Incognito | — | 2008 | GNU General Public License |
| Incompatible Timesharing System | Massachusetts Institute of Technology | — | GNU General Public License v3.0 or later |
| INK | — | — | — |
| INMOS | Institute of Electronic Control Machines | — | — |
| Inquisitor | — | — | GNU General Public License |
| Instant WebKiosk | — | — | — |
| INtime | — | — | — |
| IOS 10 | Apple Inc. | 2016 | end-user license agreement |
| IOS 11 | Apple Inc. | — | end-user license agreement |
| IOS 12 | 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 8 | Apple Inc. | 2014 | Apple Public Source License |
| IOS 9 | Apple Inc. | 2015 | end-user license agreement |
| IPadOS 15 | Apple Inc. | — | — |
| IPadOS 16 | Apple Inc. | 2022 | — |
| IPadOS 17 | Apple Inc. | 2023 | — |
| IPadOS 18 | Apple Inc. | — | — |
| IS-DOS | — | — | — |
| ISIS | Intel | — | — |
| Issabel | — | — | GNU General Public License |
| ITIX | — | — | GNU General Public License |
| ITsART | — | — | — |
| Java Card OpenPlatform | IBM | — | — |
| JavaOS | Sun Microsystems | — | proprietary license |
| Jazz Linux | — | — | — |
| JNode | — | — | GNU Lesser General Public License |
| Joli OS | — | 2011 | GNU General Public License |
| Junos | Juniper Networks | — | — |
| JX | University of Erlangen–Nuremberg | — | GNU General Public License |
| K42 | IBM | — | — |
| KaeilOS | KOAN | — | — |
| Kaleidoscope | — | — | — |
| Kali Linux for Raspberry PI | — | — | — |
| KallistiOS | — | — | 3-clause BSD License |
| KDE neon | KDE | 2016 | GNU General Public License |