The goal of this clinical trial is to evaluate the treatment method of deep brain stimulation (DBS) in patients diagnosed with severe and treatment-resistant obsessive-compulsive disorder (OCD). DBS will be administered into the brain area of the bed nucleus of stria terminalis (BNST).

The main research questions are:

• Does active DBS in BNST reduce OCD symptoms (primary outcome)?
• Does active DBS in BNST reduce anxiety or depressive symptoms and improve the level of daily function?

The participants will be randomized to active or sham DBS in the BNST for six months and we will measure the outcomes before DBS surgery, and six months post-surgery. After the randomization phase, the participants will enter an open-label phase and receive continuous DBS in BNST. Yearly follow-ups will be conducted. The outcomes will be measured through interviews using validated psychometric scales.

Inclusion Criteria:

• OCD diagnosed according to the criteria in the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV).
• The patients should have received at least 16 sessions of cognitive behavioral therapy, tried three different serotonergic antidepressants in adequate doses for three months, and one serotonergic antidepressant in combination with a neuroleptic drug.
• Severe OCD symptoms: YBOCS ≥ 25 points.
• Substantial incapacity because of his/her symptoms.
• Duration of symptoms: 5 years minimum.
• Understand the consequences of participation in the study and give informed written consent.

Exclusion Criteria:

• Not being able to understand the consequences of the treatment.
• Diagnosed with intellectual disability according to DSM-IV.
• Not meeting the requirements for the neurosurgery procedure.

We will not share IPD until the study is finished and results are published.

The participant and the outcome assessor are blinded to whether the participant receives active or sham stimulation.