| Title | Publisher | Year | Language | Documents |
|---|---|---|---|---|
| Objetivo: Madrid - Parte 1 | Galo Alcolea | 1989 | Spanish | |
| Occult Connection, The | David Munden | 1993 | English | |
| On the Loose | Lucille F. Rock | ? | English | |
| On the River Styx | H. W. Williams | ? | English | |
| On the Run | Alfo Software | 1989 | English | |
| Operation Hawaii | Golden Games | 1986 | German | Solution, Map |
| Operation Thunderstorm | Softwarehaus R. Lindenschmidt | ? | German | Solution, Map |
| Oskar und der Tod | Uwe Pilz | 1997 | German | |
| Out of the Limelight | WoW Software |
1992 | English | Solution, Map |
| Out of the Study | Anssi Räisänen | 2002 | English | Solution, Map |