SRIs are considered first-line treatments for obsessive-compulsive disorder, (OCD); however, many patients continue to have significant symptoms despite an adequate trial of an SRI. Neuroimaging studies have shown that the glutamate neurological system is involved in OCD. This study will test the safety and efficacy of topiramate, a drug, which targets glutamate, in the treatment of OCD, where the OCD has been un-responsive, or partially responsive to regular SRI treatment. The patients will be randomized in a double-blind fashion to the addition of topiramate (Topomax) or placebo to the regular dose of SRI. At the end of the study, participants will enter follow-up care with the study doctor.