The I.O., or I.O. Chicago, (formerly known as "ImprovOlympic") is a theater at 3541 N. Clark Street in the Wrigleyville neighborhood of Chicago, Illinois dedicated to improvisational comedy. It was founded in the 1980s by Del Close and Charna Halpern. The I.O. approaches improvisation as an art form in and of itself; this differs from the Second City approach, where improv is used as a tool in creating sketch comedy. I.O. also concentrates on "long-form" improvisational structures, in contrast to the "short-form" or "improv game" format of Theatresports or the television show Whose Line Is It Anyway?. The I.O. signature piece is "Harold".
I.O.'s two theater spaces, the Cabaret and the Del Close Theater, run nightly shows for audiences mostly made up of teams of former students of I.O.'s training center. I.O. is also involved in the annual Chicago Improv Festival. A companion theater, I.O. West, opened in Los Angeles, California in 2000. Additionally, the Upright Citizens Brigade (UCB), which is now based in New York and runs the Upright Citizens Brigade Theatre there, was once an I.O. house team.
In 2006, I.O. began a joint venture with ComedyWorx of Raleigh, NC to create the third I.O. training center. Dubbed the I.O. South, this training center brings the same instructors that teach in Chicago and LA to spread the art of improv through the entire Southeast.