| System (Setting) | Publications | Articles |
|---|---|---|
 O.L.D. | 1 | 0 |
| Oh Cthulhu, You've Done it Again! | 1 | 1 |
 Oh, R'lyeh? | 1 | 1 |
| One of Us Becomes and Evil God! | 1 | 1 |
| One Page Cthulhu | 4 | 3 |
| One Shot RPG | 3 | 2 |
| Orbis Aerden | 1 | 11 |
| OSR | 8 | 2 |
| The Outer Presence | 6 | 3 |
| Over the Edge | 1 | 1 |