| Oath Demo | Caleb Seibert | 1999-04-18 |
| 80 |
| Obelisk Demo | ManofAction ◪ | 1998-03-14 |
| 73 |
| Ogre Caves | Skull Games | 2004-01-01 |
| 69 |
| Oh No! Not Another Star Wars Parody! | CapnKev | 1996-09-19 |
| 214 |
| Oh? | cornsalt ◪ | 2001-01-20 |
| 66 |
| Oizys (ver. 1.3) | Lachesis ◪ | 2009-12-25 | No reviews. | 85 |
| Old MZX Junk | Evil Genius ◪ | 2016-03-27 |
| 55 |
| Old School MZX Demo | shades ◪ | 2003-02-13 |
| 56 |
| OMGWTF | RoSS ◪ | 2003-07-14 |
| 71 |
| Open Source Demo | gofer-chan ◪ | 2002-04-15 |
| 63 |
| Oppo | CHEZZY ◪ | 2007-12-03 |
| 79 |
| Orb of the Twisted Demo, The | Legendd | 1997-01-17 |
| 95 |
| Ordnance (ver. 1.1) | CJA ◪ | 2014-12-31 |
| 171 |
| Outrage | Eon Entertainment | 1996-12-16 |
| 88 |
