The recommended treatment for OCD is a combination of medication and psychotherapy. How do we know that the "talk therapy" part of treatment really works?
Many studies have shown that talk therapy, including cognitive-behavioral therapy (CBT) in particular, can definitely benefit people with OCD. Some studies have examined people who take medications like SSRIs, but who are still having symptoms. The people who added CBT to their treatment plan fared better than those who took medication alone. Other studies have shown that psychotherapy can actually change the neurobiology of the brain in some people.
Sources:
Lehto SM, Tolmunen T, Joensuu M, Saarinen PI, Valkonen-Korhonen M, Vanninen R, Ahola P, Tiihonen J, Kuikka J, Lehtonen J. "Changes in midbrain serotonin transporter availability in atypically depressed subjects after one year of psychotherapy." Prog Neuropsychopharmacol Biol Psychiatry. 2008 Jan 1;32(1):229-37. Epub 2007 Aug 19.
Simpson HB, Gorfinkle KS, Liebowitz MR. "Cognitive-behavioral therapy as an adjunct to serotonin reuptake inhibitors in obsessive-compulsive disorder: an open trial." J Clin Psychiatry. 1999 Sep;60(9):584-90.
Tundo A, Salvati L, Busto G, Di Spigno D, Falcini R. "Addition of cognitive-behavioral therapy for nonresponders to medication for obsessive-compulsive disorder: a naturalistic study." J Clin Psychiatry. 2007 Oct;68(10):1552-6.