Additional Study Information: The purpose of this study is to assess the effects of a single infusion of ketamine for adolescents and young adults with obsessive-compulsive disorder (OCD). Ketamine is a FDA-approved anesthetic medication that can possibly cause a rapid decrease in OCD symptoms. To be eligible to participate, patients need to have already tried at least one SRI medication and attempted a course of cognitive-behavioral therapy (CBT), unless unable to access or tolerate, with little or no change in OCD symptoms. The study involves: A screening visit, including a free, thorough diagnostic evaluation, in order to determine if the patient is eligible for the study; A one-time, low dose of intravenous ketamine administered at the Columbia University Medical Center; Phone check-ins and in-person visits for 2 weeks after the ketamine infusion; Three months of treatment for OCD including medication and/or cognitive-behavioral therapy at no cost.