 In this randomized controlled trial RCT the investigators experimentally test if patients with Obsessive-Compulsive Disorder OCD who have received treatment with exposure and response prevention ERP, but either relapsed or not responded, profit from the combination of concentrated exposure based treatment cET and the NMDA-agonist N-methyl-d-aspartate d-cycloserine DCS, targeting fear relevant areas in amygdala and pre-frontal cortex. 