First line treatments fo Obsessive Compulsive Disorder (OCD) are Selective Serotonin Recapture Inhibitors (SSRIs) and Cognitive Behaviour Therapy (CBT) including exposure with response prevention. The aim of the present study is to evaluate the clinical efficacy of GCBT and SSRIs for OCD patients. Other clinical trials have compared these treatments, but with OCD patients without any other psychiatric disorder. In this study patients with current age between 18 and 65 years, with YBOCS score of at least 16 and psychiatric comorbidities will be not excluded. Exclusion criteria will be: OCD secondary to brain trauma, stroke or malformation; current abuse of alcohol or other psychoactive substance, current presence of psychotic symptoms, suicidal risk, psychiatric or clinical comorbidity that might get worse with the medications used in the trial. So, the present study investigates the efficacy of these treatments for a heterogeneous OCD population, trying to identify if the usual treatments are efficient when applied in the public health system that treat not just patients with only OCD diagnosis. Patients will be randomized for GCBT and SSRI, and after treatment will be evaluated by researchers blind to the treatment received.