Olivia Olson (born May 21, 1992) is an American actress, singer-songwriter, and screenwriter, largely known for her voice roles as Vanessa Doofenshmirtz in Phineas and Ferb and Marceline the Vampire Queen in Adventure Time. She also played the character of Joanna in the 2003 film Love Actually and its 2017 short sequel Red Nose Day Actually.

Along with this, she is also a writer, having written the Boom! Studios graphic novel Marcy and Simon, co-authored Adventure Time: The Enchiridion and Marcy's Super-Secret Scrapbook, and contributed writing to the best-seller Adventure Time Encyclopedia for Cartoon Network. In 2023, she began writing for the Phineas and Ferb revival.

Olson was adopted at birth and raised in Los Angeles, California. She lives with her father, Martin Olson, mother Kay Olson, and brother Casey Olson. Olson graduated from Agoura High School in 2010. She is of Afro-Jamaican descent through her biological father.

Olson has appeared as a singer and actress on television shows and in live theatre, including Comedy Central Stage, the HBO Theater, and The Fake Gallery.

Olson moved into the mainstream with her role as Joanna Anderson in the 2003 holiday film Love Actually, singing the song "All I Want for Christmas Is You". Her singing was praised, with director Richard Curtis stating in the commentary in the music section of the Love Actually DVD that Olson's singing was so perfect, they were afraid the audience would not believe that a ten-year-old could really sing the way she did and would assume she was lip-synching. They had to train her so her singing would sound more believable, and add the sounds of inhalations to the track. This led to guest spots on The Ellen Show, The Tracy Morgan Show and singing with stars Jack Black and Kristin Chenoweth. Olson also posted original songs on her YouTube account. In 2017, Olson reprised her role as Joanna in Red Nose Day Actually, a short film which is a sequel to Love Actually.

From 2008 to 2015, Olson voiced the recurring character Vanessa Doofenshmirtz in the Disney animated series Phineas and Ferb along with actor Thomas Sangster (Ferb Fletcher), who played Joanna's love interest Sam in Love Actually. Olson said that she and Sangster talked "about the whole Ferb-Vanessa situation 'cause it's pretty funny."

In Phineas and Ferb, she sings a number of songs, including the duet "Busted!" with Ashley Tisdale as well as popular solo songs, including "I'm Me", "Not So Bad a Dad", "I'm Lindana and I Wanna Have Fun", "Happy New Year" and many others, including her song in the Phineas and Ferb Christmas Vacation special "Got That Christmas Feeling". She has written songs for Phineas and Ferb and has written and recorded her own songs with music producers Rick Nowels, Camara Kambon and Hollywood jazz great Rob Mullins, while Olson's father wrote "over 200 songs for the series" and various episodes. In 2011, Olson voiced Vanessa in a deleted scene of Phineas and Ferb the Movie: Across the 2nd Dimension, which included a musical number, but sang a song for the film "I Walk Away".

Her role as Vanessa led to her role as Marceline the Vampire Queen in Adventure Time. During the show's production, the show creator, Pendleton Ward, contacted Olson's father, Martin Olson, then writing for Phineas and Ferb, asking if he knew who played Vanessa in the show. He was not aware, at the time, that Martin was Olson's father. She would also voice Marceline in the 2012 video game, Adventure Time: Hey Ice King! Why'd You Steal Our Garbage?!!.