| Omniscient Reader's Viewpoint | — | — | — |
| On Air Dekinai! | January 10, 2022 | — | 12 |
| On The Edge | February 5, 2021 | — | — |
| Onaji Semi no Someya-san ga Sexy Joyū datta Hanashi | July 5, 2026 | — | — |
| Once Upon a Time | — | — | — |
| Once upon a time in silver showers kkabi? | — | — | — |
| Once Upon a Time... Life | December 10, 1986 | Canal+ | 26 |
| Once Upon a Time... Man | 1978 | France Régions 3 | 26 |
| Once Upon a Time... Planet Earth | December 22, 2008 | France 3 | 26 |
| Once Upon a Time... Space | October 2, 1982 | France Régions 3 | 26 |
| Once Upon a Time... The Americas | 1991 | Canal+, France 3 | 26 |
| Once Upon a Time... The Discoverers | January 3, 1994 | France 3 | 26 |
| Once Upon a Time... The Explorers | December 10, 1996 | France 3 | 26 |
| Once Upon a Time... The Objects | — | — | — |
| Once Upon a Witch's Death | April 1, 2025 | — | 12 |
| One Outs | October 8, 2008 | — | 25 |
| One Piece | December 12, 1999 | Fuji Television | 1,169 |
| One Room | January 11, 2017 | — | — |
| One Week Friends | April 6, 2014 | Tokyo MX, AT-X, Nippon BS Broadcasting, TV Aichi, MBS, ANIPLUS | 12 |
| One-Punch Man | October 4, 2015 | TX Network | 24 |
| One-Room TA | August 28, 2026 | — | 7 |
| One: Kagayaku Kisetsu e | August 10, 2001 | — | 4 |
| Oneechan ga Kita | January 8, 2014 | — | 12 |
| Onegai AiPri | April 5, 2026 | — | — |
| Onegai My Melody | April 3, 2005 | — | — |
| Onegai My Melody Kirara | April 6, 2008 | — | 52 |
| Onegai My Melody Sukkiri | April 1, 2007 | TX Network | — |
| Onegai My Melody: Kuru Kuru Shuffle | April 3, 2005 | TX Network | — |
| Onegai! Samia-don | April 2, 1985 | NHK General TV | 78 |
| Ongaku Shōjo | July 6, 2018 | — | 12 |
| Ongezellig | July 20, 2018 | — | 6 |
| Oni Chichi | October 30, 2009 | — | 2 |
| Oni Chichi 2 | October 29, 2010 | — | 2 |
| Oni: Thunder God's Tale | October 21, 2022 | — | 4 |
| Oniichan dakedo Ai sae Areba Kankeinai yo ne! | October 5, 2012 | AT-X | 12 |
| Onimai | January 5, 2023 | — | 12 |
| Onipan! | April 11, 2022 | TX Network | 12 |
| Only You: Viva! Cabaret Club | — | — | — |
| Onmyo Kaiten Re:Birth Verse | July 3, 2025 | — | 12 |
| Onmyoji | November 28, 2023 | — | 13 |
| Onmyō Taisenki | September 30, 2004 | — | 52 |
| Onna no Ko tte | February 5, 2015 | — | 4 |
| Onsen Yōsei Hakone-chan | October 4, 2015 | — | 13 |
| Onyankopon | October 6, 2017 | — | — |
| Onyx Equinox | November 21, 2020 | Crunchyroll | — |
| Oopsie Niki | — | — | — |
| Ooya-san wa Shishunki! | January 10, 2016 | — | 12 |
| Open Season: Call of Nature | November 3, 2023 | Family Channel | — |
| Operation Han-Gyaku-Sei Million Arthur | October 25, 2018 | — | 24 |
| Operavox | — | — | 6 |
| Opus Colors | April 7, 2023 | — | 12 |
| Orange | July 3, 2016 | Tokyo MX | 13 |
| Orange Moo-Cow | September 5, 2018 | — | 208 |
| Orb: On the Movements of the Earth | October 5, 2024 | NHK General TV | 25 |
| Ore to Yu Nii! | — | — | — |
| Ore wa Chokkaku | January 5, 1991 | — | 36 |
| Ore wa Teppei | September 12, 1977 | — | 28 |
| Ore, Tsushima | July 3, 2021 | — | 12 |
| Oreimo | February 22, 2011 | — | 4 |
| Oreimo | October 3, 2010 | — | — |