tDCS and Symptom Provocation in Treatment-Resistant OCD

Inclusion Criteria: * Diagnosis of Obsessive-Compulsive Disorder (OCD) according to DSM-5, with a duration ≥ 2 years. * Good insight, defined as a BABS score ≤ 18. * Chronic OCD, with either a Y-BOCS total score \> 20 or a subscale score \> 15. * Treatment-resistant OCD, defined as: * failure of ≥ two adequate trials of SSRIs, or * failure of one SSRI plus augmentation with an atypical antipsychotic, * and/or insufficient response after ≥ one year of cognitive-behavioural therapy. * Stable pharmacological treatment (fixed antidepressant dose) for at least 12 weeks prior to inclusion, with no significant improvement during this period. * Age 18 to 70 years. * Ability to provide informed written consent. * Affiliation with the French social security system. Exclusion Criteria: * Women of childbearing potential without effective contraception, or those unwilling to maintain abstinence or contraception. * Pregnancy or breastfeeding. * Compulsory psychiatric hospitalisation. * Legal guardianship or curatorship. * Current DSM-5 Axis I disorder other than OCD (schizophrenia spectrum, bipolar disorder, substance use). * Current major depressive episode (MADRS \> 21). * Suicide risk (MADRS item 10 \> 3). * Dermatological lesions at electrode placement sites. * History of traumatic brain injury. * Presence of intracranial metal, pacemaker, or epilepsy. * Emergency situations or inability to provide informed consent.