Olivia Cathrine Hack (born June 16, 1983) is an American actress and voice actress, best known for providing the voice of Rhonda Wellington Lloyd in Nickelodeon's Hey Arnold!, and for playing Cindy Brady in the 1990s theatrical Brady Bunch films. She has also done voice work for Fillmore!, Bratz as Cloe, Family Guy, Blood+, and Avatar: The Last Airbender as Ty Lee. She appeared in Star Trek Generations, Party of Five and Gilmore Girls.
As a child actress, Hack appeared in her first commercial spot when she was eight months old.
Entertainment Weekly noted the authenticity of Hack's reprisal of Cindy Brady for the 1990s Brady Bunch films, and the New York Daily News made positive comparisons of Hack's work to that of Susan Olsen's Cindy from the original television series.
In both 2000 and 2001, Hack received Young Artist Awards nominations for 'Best Performance in a Voice-Over: TV/Film/Video' for her work in the series Hey Arnold