Obsessive-compulsive disorder (OCD) is a severe psychiatric condition with only limited response to current first-line treatments, comprising pharmacotherapy and cognitive-behavioural therapy. Repetitive transcranial magnetic stimulation (rTMS) is a relatively new technique which has shown great promise in the treatment of refractory mood disorders, and schizophrenia, by alteration of brain activity. Previous work has demonstrated altered cortical excitability in OCD and preliminary studies have suggested that rTMS may have therapeutic potential in OCD. This pilot study will investigate the effectiveness of rTMS for treatment-refractory OCD, and establish appropriate treatment parameters. In addition, mechanisms whereby rTMS exerts its therapeutic effect will be explored using TMS to evaluate cortical inhibition and measures of cognitive biases and processing.