In this study we propose to investigate the effectiveness of rTMS treatment in refractory OCD. Although ideal treatment parameters need to be established, the above literature suggests that high frequency stimulation of the dorsolateral prefrontal cortex is likely to be beneficial. Bilateral stimulation will be used; our previous work has demonstrated this method to be safe and well tolerated. Moreover patients will be assigned in randomized double-blind fashion to receive either active or sham TMS, allowing for more definitive conclusions to be drawn. We also propose to explore whether the induction of CI mediates the therapeutic effects of rTMS on OCD symptoms.