| Series | Created by | Games | Genre |
|---|---|---|---|
| Oddworld | Oddworld Inhabitants | 6 | platform game |
| Ongeki | Q122741 | 1 | rhythm game |
| Operation Flashpoint | Codemasters, Bohemia Interactive | 4 | tactical shooter, first-person shooter, military simulation |
| Oribiland | Jacob the Orange Yoshi | — | boss game, action game |
| Osu! Tatakae! Ouendan | iNiS | 3 | rhythm game |
| Otome Series | Ensemble | 1 | eroge, visual novel |
| OutRun | Sumo Digital, Sega AM2, SIMS Co., Ltd. | 2 | racing video game |