Series with the first letter - "O"
- Once Upon a Time (2011–2018)
- Once Upon a Time in Wonderland (2013–2014)
- Once Upon a Time... But Not Anymore (2022–…)
- Onder Vuur (2021–…)
- One Big Happy (2015)
- One Big Recipe (2021–…)
- One Day at a Time (2017–…)
- One Day at a Time (1975–1984)
- One Foot in the Grave (1990–2001)
- One Lane Bridge (2020–…)
- One Night in... (2020–…)
- One Night Stand (2005–…)
- One Night Stand (1989–1992)
- One Of Us Is Lying (2021–…)
- One Piece: Wan pîsu (1999–…)
- One Punch Man: Wanpanman (2015–…)
- One Strange Rock (2018–…)
- One Thousand and One Nights (2012–…)
- One Tree Hill (2003–2012)
- Oni: Thunder God's Tale (2022–…)
- Onimai: I'm Now Your Sister! (2023–…)
- Only Connect (2008–…)
- Only Fools and Horses.... (1981–2003)
- Only Jokes Allowed (2022–…)
- Only Murders in the Building (2021–…)
- Only One Person (2021–…)
- Open Heart (2015)
- Operación Marea Negra (2022)
- Operation Wild (2010–…)
- Orange Is the New Black (2013–…)