| OOLO (2025) | 83,193 | 4 |
| oOo: Ascension (2018) | 26,262 | 106 |
| Ooops! 2 (2021) | 193,219 | 15 |
| Oops! Prank Party (2010) | 351,848 | 59 |
| Oops!!! I Slept With Your Mom (2018) | 1,032 | 5 |
| Oops!!! Puzzles!!! (2019) | 0 | 0 |
| Oopz-Oofs (2024) | 480 | 3 |
| The Ooze (1995) | 30,128,522 | 888 |
| Ooze Keeper (2024) | 1,609 | 6 |
| Ooze: The Great and Powerful (2025) | 91,373 | 4 |
| Oozi: Earth Adventure (2013) | 0 | 0 |
| Oozing Islands (2021) | 173 | 2 |
| OPA: Super Janken (2025) | 0 | 0 |
| Opal's Quest (2006) | 3,961 | 8 |
| Opaldune (2024) | 2,741 | 45 |
| Opaline (2017) | 61,022 | 26 |
| Opaloid Kingdom (2022) | 2,447 | 66 |
| Open Air (2025) | 4,968,665 | 751 |
| Open At Nine (2023) | 7,326 | 6 |
| Open Country (2021) | 4,727,265 | 1,312 |
| OPEN DOOR (2022) | 72,537,423 | 457 |
| Open Fishing 2 (2025) | 0 | 0 |
| Open Hunting XL (2024) | 505 | 4 |
| Open Quartz (2004) | 0 | 0 |
| Open Roads (2022) | 1,557,939 | 869 |
| Open Season (2006) | 19,157,085 | 1,664 |
| Open Sorcery: Sea++ (2021) | 788 | 8 |
| Open The Chest 2 (2020) | 0 | 0 |
| Open The Gates! (2022) | 2,487,763 | 285 |
| Open World Game: the Open World Game (2019) | 576,046 | 113 |
| Open, Sesame! (1982) | 837,388 | 217 |
| OpenArena (2008) | 401,888 | 1,542 |
| OpenCity (2009) | 10,816 | 2 |
| OpenDraft (2024) | 0 | 0 |
| OpenFrag (2007) | 4,637 | 5 |
| OpenGloves (2021) | 0 | 0 |
| OpenTibia (2001) | 55,268 | 102 |
| OpenTTD (2004) | 21,415,187 | 10,053 |
| OpenTTD (2014) | 15,348 | 28 |
| OpenVR Benchmark (2019) | 15,193 | 4 |
| OpenVTT (2022) | 0 | 0 |
| OpenXcom (2014) | 1,703,323 | 2,308 |
| Opera Fatal (1996) | 26,482 | 36 |
| Operate Now Hospital - Surgery (2016) | 146,884,882 | 651 |
| Operation : ERONTA (2023) | 0 | 0 |
| Operation Abyss: New Tokyo Legacy (2014) | 502,680 | 382 |
| Operation Apocalypse (1981) | 772,337 | 572 |
| Operation Armstrong (2021) | 2,817 | 11 |
| Operation Babel: New Tokyo Legacy (2017) | 159,908 | 181 |
| Operation Blockade (2002) | 198,656 | 81 |
| Operation Body Count (1994) | 178,421 | 307 |
| Operation C (1991) | 2,402,769 | 1,188 |
| Operation Crusader (1994) | 27,207 | 33 |
| Operation Crustacean (2024) | 2,594 | 2 |
| Operation Darkness (2007) | 475,664 | 541 |
| Operation Desert Road (2017) | 0 | 0 |
| Operation Endpoint (2022) | 0 | 0 |
| Operation Europe: Path to Victory (1991) | 199,479 | 317 |
| Operation F.A.T.E. (2023) | 252 | 1 |
| Operation Flashpoint: Cold War Crisis (2001) | 9,076,132 | 3,136 |
| Operation Flashpoint: Dragon Rising (2009) | 7,107,687 | 2,525 |
| Operation Flashpoint: Elite (2005) | 243,913 | 205 |
| Operation Flashpoint: Red Hammer (2002) | 108,749 | 178 |
| Operation Flashpoint: Red River (2011) | 6,077,439 | 1,987 |
| Operation Food to Gold (2024) | 14 | 2 |
| Operation Gunship (1989) | 5,000 | 23 |
| Operation Hardcore (2016) | 7,525 | 36 |
| Operation Harrier (1990) | 3,094 | 18 |
| Operation Logic Bomb (1993) | 258,788 | 293 |
| Operation Lovecraft: Fallen Doll (2022) | 985,916 | 83 |
| Operation Marakudja (2024) | 87 | 6 |
| Operation Market Garden (1985) | 2,251,686 | 149 |
| Operation Neptune (1991) | 219,541 | 182 |
| Operation Night Strikers (2025) | 51 | 3 |
| Operation Octo (2025) | 64,635 | 39 |
