Obsessive-compulsive disorder (OCD) is a severe and debilitating anxiety disorder afflicting about 2% of the population. It is characterized by the presence of recurrent obsessions and/or compulsions that are time consuming and cause marked distress and/or impairment. Untreated, OCD runs a chronic and deteriorating course. According to the World Health Organization, OCD is among the top 10 leading causes of disability worldwide. Examination of non-medicinal treatments for OCD has focused on two distinct treatments: exposure and response prevention (ERP) and cognitive therapy (CT), often combined into an integrated cognitive-behavioural treatment (CBT). CBT is considered the first line psychological treatment for OCD, with estimates of response rates of between 70-80%. However, there are a significant number of treatment non-responders and the majority of responders are still left with impairing residual symptoms. One area of investigation that has shown potential benefit for general mood and anxiety disorders has been mindfulness-based interventions (MBIs), yet the potential benefits of MBIs in OCD has been largely unexamined, except for several small preliminary studies that show clinical promise. The purpose of this study is to examine the clinical benefits of a standardized MBI treatment for OCD in a large-scale, multi-site randomized controlled trial. The results of this study will directly determine whether Mindfulness can be considered an effective treatment for OCD. 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 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 biological factors influence psychological treatment outcomes.