Additional Study Information: The purpose of this pilot research study is to test the effects of a medication called nabilone (Cesamet) in adults with obsessive-compulsive disorder (OCD). Participants will receive either nabilone on its own, or nabilone in combination with a form of cognitive-behavioral therapy (CBT) called exposure and response prevention (EX/RP). Nabilone is a synthetic cannabinoid and acts on the brain's "endocannabinoid system," which has been hypothesized to play a role in OCD. Nabilone is approved by the FDA for the treatment of chemotherapy-induced nausea and vomiting. It is not FDA-approved for treating OCD.