| Title | Author | KB | Genre |
|---|---|---|---|
| Oasis: Part One | WiL | 103.16 | Puzzle |
| Object Land | Joe McManis | 2.62 | Toolkit |
| Octagon | wayward | 52.83 | Adventure, Puzzle |
| Oddworld | Benco | 33.74 | Platformer, Puzzle |
| Oddworld SE | Benco | 33.93 | Platformer, Puzzle |
| Office man | Jimmy Young | 26.98 | Adventure |
| Oh no! Computer generated Puzzles! | kristomu | 6.7 | Experimental, Puzzle |
| Oh the Horror | ZZDevelin | 23.55 | Other |
| Oh What a Day | Leamas | 27.94 | 24HoZZT, Contest |
| Oh, Gee! Issue #1 | CLAbles, LMAbles | 17.5 | Magazine |
| OIL | dave2 | 50.83 | Action, Adventure |
| Oj-If Demo | Miakar | 32.18 | Action, Adventure, Demo, Puzzle |
| Oj-lf demo | Jhello73 | 29.29 | Adventure, Demo |
| Oktrollberfest 2020 Ceremony | KKairos, WiL | 87.64 | Art, Contest, Oktrollberfest |
| Old Man Keesh: Preview of A New Yoshi Game | BigtimeDudeProductions | 1.04 | Demo |
| Old Shit | Ando | 72.66 | Incomplete |
| Omega | Ben | 0.76 | Music |
| Omega Base | Omega | 32.5 | Adventure |
| Omega Demo | Sephiroth X | 6.13 | Demo |
| Omega Force 1.5 | Ringworm | 35.77 | Adventure |
| Omega Lock | Masamune | 1.04 | Utility |
| Omega Second | Recluse | 18.01 | Action, Adventure |
| Omega Wars: Alien Invasion | Unknown | 16.86 | Adventure |
| omg lol | #cactusware, maladroit | 16.73 | Cameo |
| Omidon's Downfall (Preview Version) | DarkHorse | 48.14 | Adventure, Demo, Puzzle, RPG |
| On A Distant Moon | Zephyr | 84.41 | Action, Adventure, Puzzle, Sci-Fi, Space |
| On the Wire | Draco, Zenith Nadir | 27.45 | Action |
| One Board Adventures | Quack6754 | 33.74 | Action, Compilation |
| One Day At School | Alex Arrowsmith | 10.39 | Adventure |
| one fine day | Wiki | 5.1 | Action, Comedy |
| Only a boy named David | kev-san | 3.79 | Adventure, Edutainment |
| Operation Blue Shield | Ross Werner | 14.58 | Adventure, Beta |
| Operation Blundabo | _ry0suke_ | 15.42 | Cinema |
| Operation Enigma | Kevin Denocourt | 6.63 | Adventure |
| Operation: Blue Fox | Daniel Hutson | 14.73 | Action, Shooter |
| Operation: Cobra | Master Raze | 62.22 | Action |
| Operation: Demon Hunter | King Bat | 4.99 | Adventure, Demo |
| Operation: GAMMA VELORUM | Quantum P. | 41.03 | RPG, Sci-Fi |
| Operation: GAMMA VELORUM, Part 2 | Quantum P. | 51.07 | RPG, Sci-Fi |
| Operation: GAMMA VELORUM, Part 3 | Quantum P. | 47.48 | RPG, Sci-Fi |
| Operation: GAMMA VELORUM, Part 4 | Quantum P. | 69.44 | RPG, Sci-Fi |
| Operation: Mother Assault | Robojim | 11.55 | Shooter |
| Operation: Mother Assault (v2.0) | Robojim | 11.39 | Shooter |
| Operation: MOUSE | Master Raze | 39.07 | Adventure, Comedy |
| Operation: OBRIAN | Chase Bramlage | 48.72 | Action |
| Operation: OBRIAN 2 | Chase Bramlage | 104.68 | Action |
| Oracle: World of Infinite Possibility | WiL | 15.11 | Cinema, Engine |
| Orb Holders 1 | Skull Dragon | 38.72 | RPG |
| Orb ZZT Issue #1 | Chris Johnson, Justin Camarata, Tim Williams | 11.82 | Magazine |
| Orc Killer: Blood-Slave of Gargax | Anna Anthropy | 13.78 | Art |
| Orc Killer: Blood-Slave of Gargax 2.0 Demo | Anna Anthropy | 61.57 | Comedy, Demo, Fantasy, Incomplete, Parody, RPG |
| Orc Wars | Unknown | 6.02 | Adventure |
| Oreo The Hamster | Unknown | 9.2 | Shooter |
| Orgy the FUCK of your life | Unknown | 4.91 | Other |
| The Origin of Ultrasoft | BIll Meeks | 7.01 | Adventure |
| Original ZZT Spoof Part One: Town Of ZZT | Tenguman2k | 58.42 | Comedy |
| Osk's Opus: The Rise, Fall, and Destruction of Urth, Part I (Demo) | WiL | 19.34 | Cinema, Demo |
| Out Break! | Eric Moore, Patrick Shires | 14.72 | Action |
| Outer Emergency | Viovis Acropolis | 31.55 | 24HoZZT, Contest |
| Outlaw | Nivek | 33.17 | RPG |
| Outside Influence (Modified) | El Capitan, Kosh, Zenith Nadir | 28.87 | 24HoZZT, Contest, Mod |
| Outskirts of Zweldronia! (Project Dump) | Matthew Charlap | 65.64 | Adventure, Incomplete |
| Overflow (10/31/1996) | Luke Drelick | 97.14 | Action, Adventure |
| Overflow 2.0 | Luke Drelick | 97.27 | Action, Adventure |
| Own of ZZT | Dr. Dos | 25.6 | Parody, Puzzle |
| Ownage Triangle | Aetsch, HenchSOD Kracken, Zenith Nadir | 78.41 | Art |