Cognitive Behavioural Therapy (CBT) is a first line psychological treatment for OCD with estimated response rates of between 70-80%. However, there are still a substantial number of treatment non-responders and the vast majority of responders are left with impairing residual symptoms. One new area of investigation that has shown potential benefit for the treatment of OCD has been mindfulness-based interventions (MBIs). This emerging literature provides proof of principle for the delivery and potential efficacy of MBIs for OCD, particularly as an adjunct to CBT, with all studies reporting large (d \> .80) treatment effects. To date, only our teams' small, non-controlled pilot study (N=19) has shown promise for a MBI as a stand-alone treatment for OCD separate from CBT (d = 1.10). However, all results to date for MBI treatments for OCD must be interpreted with caution as they have utilized small, underpowered sample sizes, have not included control groups, and failed to control for potentially confounding variables (e.g., medications, self-initiated exposures). The proposed four-year, two-site randomized controlled trial at two of Canada's leading OCD treatment centres (Sunnybrook Health Sciences Centre and St. Joseph's Healthcare Hamilton) that have been involved in developing and piloting MBI treatments for OCD will aim to test whether a structured, Mindfulness-Based Cognitive Therapy (MBCT) intervention for OCD is an efficacious treatment for a large, heterogeneous sample of DSM-5 (Diagnostic Statistical Manual of Mental Disorders 5th edition) diagnosed OCD participants. The primary aim of this proposed study is to examine whether a developed and piloted MBCT treatment for OCD is efficacious when compared to a waitlist condition and to explore its efficacy compared to CBT treatment for OCD. A secondary aim of the study is to examine the putative treatment mechanisms of MBCT. Past research examining treatment mechanisms in CBT for OCD have focused on the reduction of obsessional beliefs, anxiety sensitivity, and safety behaviours in producing better outcomes. In contrast, the hypothesized mechanisms in MBI treatments include the development of mindfulness skills such as acting with awareness, non-reactivity to and non-judging of inner experience, and reduction in mind wandering. The large-scale, powered RCT (randomized control trial) design will permit the examination of whether MBCT and CBT include overlapping or distinct treatment mechanisms. Treatment outcomes will be assessed with standardized measures of OCD symptomatology, general measures of mood and anxiety, a reliable and valid EEG measure to assess cognitive changes across treatment. If this study can demonstrate that a short-term mindfulness intervention can significantly reduce the suffering associated with OCD, then the findings could easily be translated into routine clinical care in and out of Canadian hospital settings. Results of this study will also potentially add to our understanding of the mechanisms that drive OCD symptoms, improve our knowledge of psychological treatment mechanisms, and elucidate how genetic/biological factors influence psychological treatment outcomes.