| Operation Pig (2018) | 624,138 | 25 |
| Operation Purge (2026) | 0 | 0 |
| Operation Red Dragon (2017) | 414 | 5 |
| Operation Sabotage (1982) | 9,923 | 7 |
| Operation Secret Storm (1991) | 111,914 | 55 |
| Operation Shadow (2004) | 163,596 | 69 |
| Operation Space Force (2024) | 26 | 7 |
| Operation Spacehog (2000) | 2,681 | 6 |
| Operation Stealth (1990) | 840,773 | 222 |
| Operation STEEL (2022) | 52,536 | 83 |
| Operation Thunderbolt (1988) | 663,912 | 408 |
| OPERATION TIGER (1998) | 17,621 | 11 |
| Operation Valor (2023) | 74,101 | 101 |
| Operation Warcade VR (2017) | 612,627 | 110 |
| Operation Wolf (1987) | 3,204,749 | 1,479 |
| Operation Wolf 3 (1994) | 140,031 | 109 |
| Operation Wolfsburg (2010) | 68,701 | 41 |
| Operation WOW (2011) | 4,324 | 14 |
| Operation7 (2008) | 18,501,905 | 3,931 |
| Operation7: Showdown (2020) | 172,673 | 237 |
| Operation: Cleanstreets (1988) | 24,228 | 32 |
| Operation: Desert Storm (1991) | 341,367 | 112 |
| Operation: Eco-Nightmare (1997) | 8,938 | 18 |
| Operation: Harsh Doorstop (2023) | 33,538,423 | 3,480 |
| Operation: Inner Space (1994) | 33,185 | 22 |
| Operation: Matriarchy (2005) | 102,384 | 148 |
| Operation: Pinkeye (2022) | 1,661 | 21 |
| Operation: Polarity Hook (2019) | 80 | 3 |
| Operation: Polygon Storm (2024) | 160,188 | 128 |
| Operation: Tango (2020) | 29,481,812 | 6,730 |
| Operation: VICUS (2022) | 19 | 1 |
| Operation: Vietnam (2007) | 83,050 | 23 |
| Operation: Weather Disaster (1995) | 51,633 | 14 |
| The Operational Art of War III (2006) | 37,099 | 65 |
| The Operative: No One Lives Forever (2000) | 4,212,355 | 1,557 |
| The Operator (2024) | 126,884,509 | 17,724 |
| Operator 41 (2019) | 6,687 | 30 |
| Operator Ace's Simple Infinite Survival (2024) | 19 | 2 |
| Operator Unknown (2026) | 0 | 0 |
| Operator8 (2022) | 6,200 | 45 |
| Operator: Observation (2026) | 0 | 0 |
| Operencia: The Stolen Sun (2019) | 2,703,521 | 2,715 |
| Operius DX (2025) | 0 | 0 |
| OPG: Clan Wars (2024) | 1,294 | 7 |
| Ophidian Wars: Opac's Journey (2010) | 416 | 4 |
| Opollo (2025) | 0 | 0 |
| Opoona (2007) | 626,939 | 693 |
| OPPAI Ero App Academy Bigger, Better, Electric Boobaloo! (2023) | 118,185 | 3 |
| OPPAI Succubus Academy Sucky and Busty, Demonic and Lusty! (2024) | 117,852 | 2 |
| Oppaidius Desert Island! (2021) | 6,529 | 1 |
| Oppaidius Summer Trouble! (2018) | 17,574 | 3 |
| Oppidum (2025) | 47 | 1 |
| Optika (2016) | 92,778 | 14 |
| Opus Building (2026) | 0 | 0 |
| Opus Castle (2021) | 747,841 | 1,931 |
| Opus Cortex: A Nonogram Puzzle Story (2025) | 0 | 0 |
| Opus Magnum (2017) | 3,110,943 | 2,244 |
| Opus: Echo of Starsong (2021) | 1,854,988 | 2,337 |
| Opus: Rocket of Whispers (2017) | 618,784 | 942 |
| Opus: The Day We Found Earth (2015) | 810,268 | 592 |
| Oracle of Forgotten Testament (2018) | 10,817 | 41 |
| Oracle Trials (2021) | 10,074 | 22 |
| The Oracle's Cave (1984) | 36,322 | 27 |
| Orake (2016) | 20,801 | 11 |
| The Orange Box (2007) | 12,106,257 | 1,081 |
| Orange Cast: Sci-Fi Space Action Game (2021) | 231,651 | 125 |
| Orange Life (2025) | 0 | 0 |
| Orange Moon (2017) | 30,092 | 47 |
| Orange Santa (2021) | 0 | 0 |
| Orangeblood (2020) | 393,819 | 420 |
| Orb and the Stars (2021) | 68 | 1 |
| Orb of Aeternum (2024) | 4,401 | 5 |
| Orb of Creation (2022) | 619,906 | 367 |
| Orb Of The Watcher (2024) | 167 | 7 |
| Orb Overload (2021) | 42 | 1 |
