Clinical significance analyses of controlled studies comparing Exposure and Response Prevention (ERP) and Cognitive Therapy (CT) in the treatment of Obsessive-Compulsive Disorder (OCD) are scarce. The objective of this study is to compare the clinical efficacy of ERP and CT for OCD patients, and the usefulness of each in changing dysfunctional beliefs and thought control strategies at post-treatment and at a one-year follow-up. The two treatments were delivered on the basis of a routine clinical practice in a public-mental health service. Thirty-three OCD patients were randomly assigned to ERP or CT, and 29 completed the treatments (13 in ERP and 16 in CT). The ERP applied was in vivo, gradual and therapist-guided. The CT was designed to challenge all the cognitive domains considered relevant for OCD, using cognitive techniques. The improvement and recovery rates (YBOCS) were slightly superior for CT than for ERP (ERP: 69.23% and 61.53%, respectively; CT: 81.25% and 68.75%, respectively). These therapeutic outcomes were maintained after the two treatments: at the one-year follow-up, 53.85% of the treated patients remained free of symptoms in ERP, and 65.5% in CT. Finally, the two treatments were equally effective in modifying dysfunctional beliefs, and the outcomes at the end of the treatments were maintained, or even increased, one year later.