Additional Study Information: This study is investigating how the brains of people with OCD may change with treatment. The study uses Magnetic Resonance Imaging (MRI) to scan the brain, and the treatment used is a form of Cognitive-Behavioral therapy consisting of Exposure and Ritual Prevention (EX/RP). Participants will undergo a scan, which lasts for approximately 2 hours and takes images (pictures) of your brain. Participants will then be offered 17 sessions of EX/RP treatment over 9-10 weeks, and will undergo a second scan at the end of this treatment. There is no radiation exposure risk from the MRI scan, and the images are not used for clinical or diagnostic purposes. All information will be kept completely confidential.