<qa>  What is the name of the group that is referred to the specialist health care?A total of 160 relapsed and non-responding OCD-patients, treated with exposure and response prevention ERP in the specialist Health care in Norway, are planned to participate in the study: Translation approach to the understanding and treatment of Obsessive-Compulsive Disorder OCD. Can D-cycloserine DCS enhance and stabilize the treatment-response in relapsed and non-responding OCD-patients? A randomized, double-blind, placebo-controlled national study. Estimated start of inclusion is November 2015 and the inclusion period lasts for maximum 2 years. All participants are referred to the specialist health care and are pre-screen by the local OCD-team. Before inclusion, the patients will fill in a number of questionnaires on-line and all patients who meet the inclusion criteria will be invited to participate. Before the patient signs the informed consent form the patients will receive written and oral information about the study as well as watch a video describing the trial and what participation will imply. Before the 4-day concentrated exposure based treatment cET starts, the patients will undergo a clinical assessment interview by the local OCD-team, and will watch another video describing the trial in detail. They will also be assessed by SCID-I Structured Clinical Interview for DSM 5 as well as Y-BOCS Yale-Brown Obsessive Compulsive Scale interview by independent and specially trained psychologists. The cET is delivered in an individual group format which implies that the patient: therapist-ratio is 1:1, and that the treatment is delivered in groups of minimum 3 and maximum 6 patients. All groups are led by a specially trained cET therapist, and the local therapists are experienced OCD-therapists who are familiar with the cET format. Day 1 of the treatment 3 h consists of psychoeducation and planning of exposure tasks. Day 2 and day 3 both 8 h contact in the evening consist of individually tailored and therapist assisted exposure with a number of relevant triggers in a diversity of settings. All exposures are based on the LEaning in Technique LET, where the patients are trained to recognize when the urge to ritualize starts, and to actively approach the trigger by leaning into the anxiety. The exposures are also designed to optimize learned inhibition. In addition to individual exposure training, the group meets three times throughout the day. Day 2 and Day 3 the patient will take study medication before the exposures start. The study medication is D-Cycloserine DCS 250 mg, DCS 100 mg or placebo. In the afternoon Day 3, the patients relatives friends are invited to a 2 h lecture psychoeducation on OCD and the current treatment. Day 4 the focus is on lessons learnt as well as on planning specifying exposure tasks for the coming three weeks. Before and during the cET the patient will record data electronically via CheckWare an electronic case report form database. The patient will continue registration of obsessions and compulsions through 3 weeks post cET. One week after cET a post-assessment with Y-BOCS will be performed by the independent assessor. After 3 months, the patient is invited to an individual visit at the clinic. 1 year and 5 years post treatment, the assessment team will perform follow-up telephone interviews. 