Background and Objective: Obsessive-Compulsive Disorder (OCD) is the fourth prevalent psychiatric disorder. Despite the known first line therapies including Clomipramin and Specified Serotonin Reuptake Inhibitors (SSRI), 40% to 50% of the patients are either resistant to treatment or do not show enough response. On the other hand, some of the previous studies have shown a positive effect of Olanzapine augmentation therapy in the OCD patients but, some studies have not confirmed such effects. Therefore, we added the Olanzapine to the medical regimen of the OCD patients and studied the effectiveness of this intervention. Materials and Methods: Forty two patients with OCD according to DSM-IV-TR who were resistant to treatment with SSRI were entered to the study. 20 patients in the study group were taken 5-10 mg Olanzapine in addition to their previous drug regimen for six weeks. 22 patients in the control group received the above mentioned drug regimen without Olanzapine. Then, in order to evaluate the severity of disorder, the Yale-Brown test was done at the first, third and the sixth weeks after the study. 6 patients were withdrawn from the study because of the complications of the Olanzapine. Results: YBOCS score in the treatment group was 33.6 ± 5 and in the control group was 34.8 ± 3.3. After Olanzapine augmentation therapy the score decreased to a mean of 7.2 ± 6.1 in the treatment group and 7.1 ± 5.9 in the control group. However, there was no significant statistical difference between the two groups (P>0.001). Conclusion: The findings of this study showed that Olanzapine as augmentation, supplemental and strengthening therapy has a neutral effect in OCD patients.