| Title | Publisher | Year | Language | Documents |
|---|---|---|---|---|
| Offensive Probing | Benjamin Richard "Yahtzee" Croshaw |
1999 | English | Solution |
| Oily Deeps, The | John Cater | 1999 | English | Solution, Map |
| On the Farm | Lenny Pitts | 1999 | English | Solution |
| One Foot Down | Jason Reigstad | 1999 | English | Solution |
| One Night in the North Atlantic | Christopher Huang | 1999 | English | Solution, Map |
| Only After Dark | Gunther Schmidl | 1999 | English | Solution, Map |
| OnNoEf | Evin Robertson | 1999 | English | Solution, Map |
| Outsided | Chad Elliott | 1999 | English | Solution |