This study aims to determine if a standard, evidence-based 12-week aerobic exercise program results in significant reduction of obsessive-compulsive symptom severity and associated cognitive dysfunction as a stand-alone intervention and when combined with the first-line psychological treatment, cognitive behavioural therapy (CBT). There are 3 broad aims to this study: 1) to compare the relative efficacy of Exercise, CBT, and their combination (CBT+Ex) versus a non-treatment waitlist control (WL), 2) to examine the extent to which neuropsychological features of OCD improve following treatments, and 3) to determine the extent to which the BDNF gene and protein are a) associated with learning and cognitive factors, and b) moderate symptom and cognitive change across treatments.