From: http://www.ocfoundation.org/what-is-ocd.html

There is no proven cause of OCD. Research suggests that OCD involves problems in communication between the front part of the brain (the orbital cortex) and deeper structures (the basal ganglia). These brain structures use the chemical messenger serotonin. It is believed that insufficient levels of serotonin are involved in OCD. Drugs that increase the brain concentration of serotonin often help improve OCD symptoms.

Pictures of the brain at work also show that the brain circuits involved in OCD return toward normal in those who improve after taking a serotonin medication or receiving cognitive-behavioral psychotherapy. Although it seems clear that reduced levels of serotonin play a role in OCD, there are no laboratory tests for OCD. The diagnosis is made based on an assessment of the person's symptoms. When OCD starts suddenly in childhood in association with strep throat, an autoimmune mechanism may be involved, and treatment with an antibiotic may prove helpful.