| Only the Holly | 2018-11-21 | - |
| Only in Passing | - | - (1) |
| Only You and Me | 2024-10-01 | - |
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| Only You -Re Cross- | 2001-07-26 | 7.30 (38) |
| Only You - Sekai no Namida to Shojo no Hanashi | - | - |
| Only When You Sleep | 2024-08-22 | - (2) |
| Only One ~Sayaka Futari Dake no Kyuudoujou~ | 2003-11-28 | 5.90 (5) |
| Only One ~Minano Sensei no Poolside Lesson~ | 2003-11-28 | - (4) |
| Only One 2 ~Naomi Ai no Inori ga Todoku Toki~ | 2004-12-03 | - (3) |
| Only One 2 ~Meshimase Megumi no Sweet Apron~ | 2005-01-28 | - (4) |
| Only One 2 ~Bijin Casino Dealer Hitomi~Sei no Roulette~ | 2005-02-18 | 6.40 (8) |
| Only One + R ~Higyaku no Mermaid Minano~ | 2004-08-20 | - (3) |
| Only One | 2013-07 | 5.50 (7) |
| Only If... | 2018-11-09 | - |
| Only I Have Skills | 2023-06-26 | - (2) |
| Only Girl in High School | 2020-06-02 | - (1) |
| Only Dance! | 2025-03-26 | 6.60 (7) |
| Online Obsession | TBA | 7.40 (22) |
| Online Love | - | - (1) |
| Online Friends | - | - |
| Online @ The Perfect Time | - | - |
| Onizukin | 2011-04-27 | - |
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| Oniyarai Mura | 2007-08-17 | - |
| Oniyadori | 2020-12-07 | - |
| Onitsubaki 2 ~the end of the love~ | 2005-08-14 | - |
| Onitori+ | 2008-08-16 | - (1) |
| Onirio | - | - |
| Oningyou de Asobou | - | - |
| Oningyou Yamaa Yumashou Kudanzaka Kaiki Sousho 2 | 2024-05-06 | - (1) |
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| Onikiri-den | 2000-01-21 | - |
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| Oniichan to Issho! Happy Birthday!! | 2003 | - |
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| Oniichan Continue! | 2025-08-17 | 6.80 (62) |
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| Onii-san to Issho - Chokotto+ | 2011-02-25 | - |
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| Onii-chan, Migite no Shiyou o Kinshi Shimasu! 2 | 2017-09-29 | 5.80 (42) |
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| Onii-chan, Kiss no Junbi wa Mada Desu ka? | 2016-04-28 | 7.20 (1210) |
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| Onii-chan, Date Desu! 2 | 2019-06-29 | - |
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| Onii-chan, Asa made Zutto Gyutte Shite! | 2018-05-25 | 7.40 (511) |