Chaplin was born in Madrid to English-American actress Geraldine Chaplin and Chilean cinematographer Patricio Castilla.[2][3] She has a half-brother named Shane from her mother's previous relationship with film director Carlos Saura.[3] Her paternal grandmother, Hilda Valderrama, was a Mapuche human rights lawyer.[4] A member of the Chaplin family, she is a granddaughter of English filmmaker and actor Charlie Chaplin, and great-granddaughter of American playwright Eugene O'Neill.[5] She was named after her maternal grandmother Oona O'Neill, Charlie Chaplin's fourth and final wife.[6]
Growing up, Chaplin spent time in Spain, the United Kingdom, Switzerland and Cuba. She travelled often because of her mother's film career.[7][8] She started dancing ballet, salsa and flamenco at an early age.[8][9]
After graduating from RADA, Chaplin acted in mainly British and Spanish short and feature films. She has played alongside her mother in the feature films Inconceivable, ¿Para qué sirve un oso?, Imago Mortis and Anchor and Hope. She has had supporting roles in British and American television. She appeared as a Brazilian cage dancer in ITV's Married Single Other (2010); as Marnie Madden, the wife of main character Hector Madden, in the BBC period drama The Hour (2011–2012); as John Watson's girlfriend in "A Scandal in Belgravia", an episode of BBC's Sherlock (2012); and as Talisa Maegyr in HBO's Game of Thrones (2012–2013). She played Kitty Trevelyan, a lead character, in the BBC drama The Crimson Field (2014), and Ira Levinson's wife Ruth Levinson in The Longest Ride (2015).
In 2025, Chaplin co-starred in Avatar: Fire and Ash, directed by James Cameron, as the villainous Varang.[14][15] On casting the relatively unknown Chaplin, Cameron explained: "There was something [Chaplin] just locked into. There's a sexuality; there's a dominating psychology, and there's a lot of fury. There are a lot of layers to what she's doing there and the forces that are driving her. Oona was able to move fluidly back and forth between those in a way that I wasn't seeing with the others."[16]
↑Ross, David (28 April 2009). "Students step back in time to celebrate 75 years: Pupils and staff make it a special 1930s anniversary for Gordonstoun school". The Herald. p.3.
↑Thynne, Jane (30 July 2003). "Parents: Just like mum: Choosing the same career as a parent is very common – but not necessarily a good idea". The Guardian. p.14.
