| Title | Publisher | Year | Language | Documents |
|---|---|---|---|---|
| Of Forms Unknown | Chris Markwyn | 1996 | English | |
| On Optimism | Zach Flynn |
2005 | English | Solution |
| One Night in the North Atlantic | Christopher Huang | 1999 | English | Solution, Map |
| Over Here! | Lionel Ange |
2020 | English | Solution, Map, Hints |
| Oxphar | ERE Informatique | 1987 | French | Solution |