| Title | Year | Genre |
|---|---|---|
| O-to-i-re - Dreamcast Sequencer | 1999 | Other |
| Ogami Ichiro Funtouki - Sakura Taisen Kayou Show - Benitokage Yori | 2000 | Adventure |
| Omikron - The Nomad Soul | 2000 | Adventure |
| Omoide ni Kawaru-Kimi - Memories Off | 2002 | Adventure |
| Onsei Ninshiki Mahjong - Jarman | 2000 | Mahjong |
| Ooga Booga | 2001 | Action |
| Orange Pocket | 2004 | Adventure |
| Orion's Puzzle Collection | 2016 | Puzzle |
| Ouka Houshin | 1999 | Adventure |
| OutTrigger - International Counter Terrorism Special Force | 2001 | Action |
