She is an American singer-songwriter, actress, and writer mostly 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. Olson is also a writer, having written the Boom Comics 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.