| Oreshura | January 6, 2013 | — | 13 |
| Oresuki | October 3, 2019 | — | — |
| Oretachi Ijiwaru-kei | December 26, 2004 | — | 14 |
| Ori-Princess | — | CCTV-14 Children, Haha-Toonmax TV | — |
| Orient | January 6, 2022 | — | 24 |
| Oriental Martial Academy | July 16, 2026 | Youku | 26 |
| Ormhildur the Brave | January 3, 2026 | RÚV | 26 |
| Orson and Olivia | February 8, 1994 | Canal+ | 26 |
| Oruchuban Ebichu | August 1, 1999 | — | 12 |
| Osake ni Yowai Andō-san | — | — | — |
| Osamake | April 14, 2021 | — | 12 |
| Oscar's Oasis | February 28, 2011 | Canal+ | 78 |
| Oscar, der Ballonfahrer | 2010 | — | 26 |
| Oshare ni Naritai! Peanuts-kun | — | — | — |
| Oshi no Ko | April 12, 2023 | — | 35 |
| Oshiri Kajiri Mushi | October 7, 2012 | NHK BS Premium | — |
| Osomatsu-kun | February 5, 1966 | — | 56 |
| Osomatsu-kun | February 13, 1988 | — | 86 |
| Oswald | August 20, 2001 | Nickelodeon, Treehouse TV, Nick Jr. Channel, Tiny Living | 26 |
| Oswaldo | October 11, 2017 | Cartoon Network | 13 |
| Ot En Bilotte | October 2, 1994 | VPRO | 15 |
| Otaku Elf | April 8, 2023 | — | 12 |
| Otaku Vampire's Love Bite | — | — | — |
| Otanoshimi Anime Gekijō | April 5, 1970 | — | 26 |
| Otenba Lulu | — | — | — |
| Otherside Picnic | January 4, 2021 | — | 12 |
| Otherworldly Munchkin | — | — | — |
| Otogi Manga Calendar | May 1, 1961 | — | — |
| Otogi Zoshi | July 6, 2004 | Nippon Television | 26 |
| Otogi-Jūshi Akazukin | July 1, 2006 | — | — |
| Otoko Ippiki Gaki Daishō | September 29, 1969 | — | 156 |
| Otome wa Boku ni Koishiteru | October 8, 2006 | — | 12 |
| Otome wa Boku ni Koishiteru: Futari no Elder | — | — | 3 |
| Otome Yōkai Zakuro | October 5, 2010 | TV Tokyo | 13 |
| Otona nya Koi no Shikata ga Wakaranē! | October 5, 2020 | — | 8 |
| Our Cartoon President | February 11, 2018 | Showtime | 36 |
| Our Home's Fox Deity | April 6, 2008 | — | 24 |
| Our Last Crusade or the Rise of a New World | October 7, 2020 | — | 12 |
| Ouran High School Host Club | April 5, 2006 | Nippon Television | 26 |
| Out There | April 19, 2013 | IFC | 10 |
| Outbreak Company | October 3, 2013 | — | 12 |
| Outlaw Star | January 9, 1998 | — | 26 |
| Over Drive | April 4, 2007 | — | 26 |
| Over Drive Girl 1/6 | April 6, 2019 | — | — |
| Over the Divine Realms | 2025 | IQIYI | 17 |
| Over the Garden Wall | November 3, 2014 | Cartoon Network | 10 |
| Overflow | — | — | 8 |
| Overgeared | October 2, 2026 | — | — |
| Overlord | July 7, 2015 | AT-X | 52 |
| Overman King Gainer | September 7, 2002 | — | 26 |
| Overtake! | October 1, 2023 | — | 12 |
| Ox Tales | April 7, 1987 | TV Tokyo | 101 |
| Oyako Club | October 3, 1994 | — | — |
| Ozanari Dungeon: Kaze no Tō | September 27, 1991 | — | — |
| Ozie Boo! | April 15, 2006 | Playhouse Disney | — |
| Ozmafia!! | July 7, 2016 | — | 12 |
| Ozuma | March 16, 2012 | — | 6 |
| Ozzy & Drix | September 14, 2002 | The WB | 26 |
| Pablo the Little Red Fox | September 28, 1999 | CBeebies | 52 |
| Paboo & Mojies | May 5, 2012 | — | — |