Inclusion Criteria:  Between 21-55 years of age Physically healthy and, if female, not pregnant Able to tolerate all study procedures Able to provide written informed consent to participate Right-handed Primary diagnosis of OCD Not taking psychotropic medications or other substances likely to interact with nabilone  Exclusion Criteria:  History of any significant medical condition that may increase the risk of participation Females who are pregnant or nursing Current or lifetime history of psychiatric disorders other than OCD that may increase the risk of participation (e.g. lifetime psychosis or bipolar disorder) Current substance use disorder Positive urine toxicology or alcohol breathalyzer Any history of adverse reaction to a cannabinoid History of receiving cognitive behavior or exposure-based psychotherapy in the past 3 months History of ferrous-containing metals within the body (e.g., aneurysm clips, shrapnel/retained particles) History of claustrophobia or unable to tolerate confined spaces like an MRI