| Online @ The Perfect Time | - | - |
| Online Friends | - | - |
| Online Love | - | - (1) |
| Online Obsession | TBA | 7.40 (22) |
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| Only Girl in High School | 2020-06-02 | - (1) |
| Only I Have Skills | 2023-06-26 | - (2) |
| Only If... | 2018-11-09 | - |
| Only One | 2013-07 | 5.50 (7) |
| Only One + R ~Higyaku no Mermaid Minano~ | 2004-08-20 | - (3) |
| Only One 2 ~Bijin Casino Dealer Hitomi~Sei no Roulette~ | 2005-02-18 | 6.40 (8) |
| Only One 2 ~Meshimase Megumi no Sweet Apron~ | 2005-01-28 | - (4) |
| Only One 2 ~Naomi Ai no Inori ga Todoku Toki~ | 2004-12-03 | - (3) |
| Only One ~Minano Sensei no Poolside Lesson~ | 2003-11-28 | - (4) |
| Only One ~Sayaka Futari Dake no Kyuudoujou~ | 2003-11-28 | 5.90 (5) |
| Only When You Sleep | 2024-08-22 | - (2) |
| Only You - Sekai no Namida to Shojo no Hanashi | - | - |
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| Only You -Seikimatsu no Juliette-tachi- | 1996-01-01 | 7.10 (17) |
| Only You and Me | 2024-10-01 | - |
| Only in Passing | - | - (1) |
| Only the Holly | 2018-11-21 | - |
| Only the Roots Remain | - | - |
| OnlyDefense | - | - (1) |
| OnlyFriends | 2023-12-04 | - |
| OnlyFriends Stories: Special Agent Wexler | 2023-01-30 | - (1) |
| Onmyoji: Beyond Time | 2022-03-25 | - |
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| Onmyouji Heian Kaikoroku | 2019-02-11 | - (3) |
| Onmyouji no Kokoroe ~Sennen no Kiseki~ | 2015-11-30 | - |
| Onmyoutantei Reon | 1999-12 | - |
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| Onna Darake no Isekai ni Shoukan Sareta Hi | 2019-02-15 | - |
| Onna Doushi no Keiyaku Kekkon kara Hajimaru Koi | 2022-04-12 | - (1) |
| Onna Gari | 2008-08-23 | - |
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| Onna Hakase to Joshu-kun no Shiborare Seikatsu | 2020-02-21 | - |
| Onna Kaitou S-S-catS -Karei Naru Kaitou Inzai ni Ochiru- | 2012-09-07 | - (1) |
| Onna Kaitou Silver Cat | 2017-06-29 | 5.00 (38) |
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| Onna Kajishi ~Mazomesu ni Ochiru~ | 2022-02-11 | - (2) |
| Onna Kakutouka Ranbu | 2011-06-09 | - |
| Onna Kateikyoushi ~Joshidaisei - Yuuko no Higeki~ | 2004-12-30 | - |
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| Onna Kenshi Meru no Injoku ~Umitsukerareta Inma no Tamago~ | 2010-10-22 | - (3) |
| Onna Kentoushi Minerva ~Monster Colosseum~ | 2013-06-21 | 6.30 (75) |
| Onna Kentoushi ~Chijoku no Tougijou~ | 2015-10-03 | - |