Repetitive transcranial magnetic stimulation (rTMS) has been shown in several previous clinical trials to be an effective treatment for obsessive-compulsive disorder (OCD). However, the neural working mechanisms of rTMS in OCD are unknown, and the optimal stimulation sites have not yet been established. Our study aims to compare the clinical and neurobiological effects of three different rTMS stimulation protocols in OCD patients. 8 weeks of rTMS therapy will be delivered in combination with cognitive behavioural therapy. Multimodal neuroimaging will be carried out before and after treatment in order to demonstrate the neurobiological effects of the therapy.

Inclusion Criteria:

OCD patients:

• Age between 18 and 65
• Primary DSM-5 diagnosis of OCD
• Moderate to severe OCD symptoms (expressed as a minimum score of 16 on the Yale - Brown Obsessive Compulsive Scale (YBOCS)
• Unmedicated or stable dose of medication for at least 12 weeks prior to randomisation - with no plans to change dose during the study period
• At least 1 previous attempt at cognitive behavioural therapy (CBT) in lifetime
• At least 1 previous attempt with serotonergic medication or strong preference for non-pharmacological treatment
• Capacity to provide informed consent

Healthy controls (baseline measurements only):

• Age between 18 and 65
• Capacity to provide informed consent

Exclusion Criteria:

OCD patients:

• MRI exclusion criteria (metal in body, pregnancy)
• TMS exclusion criteria (metal in body, history of epilepsy)
• Schizophrenia, bipolar disorder, active suicidal ideation, use of antipsychotics within last 12 weeks
• previous experience with rTMS as treatment

Healthy controls:

• Current Diagnostic and Statistical Manual (DSM)-5 diagnosis
• Personal history of DSM-5 diagnosis
• use of psychotropic medications within last 12 months
• 1st degree family member with OCD
• MRI exclusion criteria (as above)
• TMS exclusion criteria (as above)