| OS95 Plus! (2026) | 0 | 0 |
| Oscar (1993) | 415,054 | 76 |
| Oscar der Ballonfahrer (2010) | 5,397 | 8 |
| Oscar's World Tour (2011) | 1,067 | 3 |
| OscarWildeCard (2023) | 1 | 1 |
| Oscuria - The world of nightmares (2021) | 566 | 5 |
| Oscuro Blossom's Glow (2025) | 0 | 0 |
| Oseong-gwa Haneum (1993) | 155 | 2 |
| OSES (2017) | 24 | 2 |
| OshiRabu: Waifus Over Husbandos (2020) | 122,227 | 115 |
| Osiris: New Dawn (2016) | 48,524,707 | 10,985 |
| Osman (1996) | 452,025,111 | 3,863 |
| Osmos (2009) | 63,132,222 | 729 |
| OSR Unhinged (2010) | 0 | 0 |
| Ossuary (2015) | 23,166 | 51 |
| Ostalgie: The Berlin Wall (2018) | 418,782 | 307 |
| Osteoblasts (2021) | 39,371 | 134 |
| Ostranauts (2020) | 3,737,971 | 2,165 |
| Ostrich Island: Escape from Paradise (2015) | 6,248,800 | 86 |
| Ostrich Runner (2004) | 42,991 | 109 |
| Ostriv (2020) | 28,597,147 | 8,455 |
| Ostrom (2024) | 116,057,725 | 15 |
| Ostron (1983) | 151 | 2 |
| osu! (2007) | 1,654,163,285 | 228,857 |
| Osu! Tatakae! Ouendan (2005) | 619,068 | 184 |
| Oswald's Adventure (2017) | 0 | 0 |
| Osyaberi! Horijyo! (2018) | 6,152 | 24 |
| Osyaberi! Puzzle Chigatan: Spot the Differences with Everyone (2020) | 2,016 | 4 |
| Otaku Engine (2022) | 233 | 1 |
| Otaku Miracles (2025) | 0 | 0 |
| Otaku no Seiza: An Adventure in the Otaku Galaxy (1991) | 473,133 | 137 |
| Otaku's Adventure (2019) | 62,145,158 | 2,172 |
| Otem's Defiance (2016) | 18,521 | 41 |
| Othello (1980) | 12,357 | 18 |
| Othello (1986) | 77,333 | 180 |
| Othello Let's Go (2020) | 241 | 11 |
| Othello World (1992) | 291,132 | 189 |
| Othello：Daynight Time Clash (2022) | 225 | 2 |
| The Other 99 (2016) | 218,454 | 90 |
| The Other Half (2018) | 13,878,639 | 619 |
| The Other Side Of The Screen (2018) | 3,900,582 | 72 |
| The other side: My own horror (2021) | 573 | 5 |
| Other World Survivors (2023) | 14,746 | 19 |
| The Other: Airi's Adventure (2019) | 977 | 5 |
| The Other: Rosie's Road of Love (2021) | 573 | 8 |
| Othercide (2020) | 13,140,695 | 4,803 |
| Otherlights (2024) | 1,418 | 12 |
| The Otherside: Realm of Eons (2009) | 12,284 | 11 |
| Otherskin (2025) | 44,194 | 87 |
| Otherwar (2023) | 41,237 | 227 |
| The Otherwilde (2022) | 1,186 | 8 |
| Otherworld Legends (2020) | 5,806,382 | 1,636 |
| Otherworld Legends (2022) | 1,767,474 | 1,028 |
| Otherworld: Omens of Summer (2013) | 68,014 | 180 |
| Otherworld: Shades of Fall (2013) | 45,302 | 128 |
| Otherworld: Spring of Shadows (2012) | 501,226 | 193 |
| Otherworldly (2020) | 2,763,357 | 292 |
| Otherworldly Air Gunship (2025) | 599 | 1 |
| Otiiz's adventure - Sushi Champ (2018) | 0 | 0 |
| Otiiz's adventure 2 (2019) | 0 | 0 |
| Otoboke Ninja Colosseum (1995) | 33,410 | 27 |
| Otocadoll (2015) | 18,422 | 52 |
| Otocky (1987) | 62,990 | 65 |
| Otogi 2: Immortal Warriors (2003) | 385,837 | 302 |
| Otogi: Myth of Demons (2002) | 822,589 | 639 |
| Otome Legends (2023) | 11,150 | 46 |
| Otome Riron to Sono Shuuhen -École de Paris (2023) | 0 | 0 |
| Otomedius (2007) | 606,911 | 667 |
| Otomedius Excellent (2011) | 1,827,128 | 640 |
| Otometeki Koi Kakumei Love Revo!! (2006) | 2,992,541 | 283 |
| Ototo (2010) | 46,230 | 7 |
| Otter Space Rescue (2019) | 13 | 1 |
| The Otter Ways (2024) | 8,667 | 3 |
| The Otterman Empire (2020) | 66,978 | 210 |
| Otti: house keeper (2020) | 2,822 | 15 |
