Obsessive-Compulsive Disorder (OCD) is a common neuropsychiatric illness beginning in childhood. Effective OCD treatments include cognitive-behaviour therapy (CBT) and medications but access to treatment is difficult and does not systematically include parents. The investigators will evaluate clinical and neural effects of Group-based Family CBT (GF-CBT), via a case-control study including: Group 1 - OCD cases receiving GF-CBT (N=90); Group 2 - OCD waitlist cases (N=90). Effects will be measured between baseline and completion of 12 GF-CBT sessions: comparing OCD severity and functioning changes between Groups 1 and 2.