The purpose of this study is to assess and compare the overall efficacy and differential effects of two interventions for patients suffering from obsessive-compulsive disorder (OCD) according to DSM-5. The interventions to be compared are detached mindfulness (DM) and cognitive restructuring (CR). Whereas the efficacy of CR for the treatment of OCD has been proven in various studies, the efficacy of DM as a single intervention so far has only been shown within a case study and within a non-clinical sample. By contrast, the efficacy of complex metacognitive protocols (of which DM constitutes one of several single interventions) has been shown multiple times. So far, however, little is known about the efficacy of DM as a single intervention in a clinical sample and the differential working mechanisms of DM vs. CR.

Whereas DM is supposed to modify metacognitions by teaching patients to solely observe their intrusive thoughts, CR aims at teaching the patients to actively question and modify their distorted interpretations of their intrusions. Since previous research has shown that symptom reduction is mediated by a change in metacognition but not by a change of distorted interpretations, the investigators hypothesize that, while expecting both interventions to be similarly effective on an overall scale, DM will lead to a change in both metacognitions and distorted interpretations whereas CR will predominantly have an effect on distorted interpretations while influencing metacognitions to a lesser extent. Further research questions address differences concerning the applicability of the two interventions in patients' everyday life and the degree to which an intense psychoeducation can already have an effect on the participants' overall symptom burden.

The intervention (both DM and CR) is spread over four sessions of 100 min each taking place within two weeks (i.e. two sessions per week) and includes intensive homework assignments for the patients. The intervention will be delivered by MSc-level clinical psychologists completing the clinical training for becoming a licenced psychotherapist in Germany ("Psychologischer Psychotherapeut"). The diagnostic assessment involves clinical interviews and a number of questionnaires and will be conducted by independent assessors who have a qualification similar to the study therapists. Assessment involves pre-, post- and follow-up assessment. Additionally, assessment comprises data collection via ecological momentary assessment (EMA) in order to measure OCD symptoms (intrusions, emotions and coping strategies), the degree to which participants apply the newly learned strategy (DM or CR) in everyday life and the amount of relief experienced from applying it. There will be one EMA before (Pre-EMA) and one after the intervention (Post-EMA). For EMA, participants receive a smartphone for four days each (Friday to Monday) and are randomly prompted ten times per day to fill in a short questionnaire. The average amount of time necessary to fill in the questionnaire is assumed to be less than 2 min, based on a sample trial with mentally healthy participants.

Participants will be randomly assigned to one of three groups: 1) DM, 2) CR, and 3) a waitlist control group. The waitlist control group will wait for two weeks (i.e., as long as the intervention in the other two groups last), before participants will be once more randomly assigned to one of the two active conditions (DM and CR), which they will then regularly participate in. Thus, all participants enrolled in the study receive one of the two treatments sooner or later. For participants who are initially assigned to the waitlist group, there will be a second pre-assessment prior to the start of the intervention.

The study is going to be sponsored by the Christoph-Dornier-Stiftung für Klinische Psychologie, which is a non-profit organization that supports research in clinical psychology and awards PhD scholarships to clinical psychologists. The study at hand is the major part of the PhD project of Christian Rupp, M. Sc., and Charlotte Juergens, M. Sc., who are currently holding a PhD scholarship by the Christoph-Dornier-Stiftung für Klinische Psychologie. The Christoph-Dornier-Stiftung für Klinische Psychologie collaborates with Department of Clinical Psychology at the University of Muenster (Westfälische Wilhelms-Universität). The The PhD project is being supervised scientifically by Principal Investigator Prof. Dr. Ulrike Buhlmann, who is a professor of clinical psychology at the University of Muenster (Westfälische Wilhelms-Universität), Germany.