Effective treatments for obsessive-compulsive disorder (OCD) usually emphasize a behavioural approach called Exposure and Response Prevention (ERP). In this treatment, patients are encouraged to face their fears repeatedly and for extended periods of time, with the help and support of a caring therapist. Although this is an approach that has been shown to work, many patients find the treatment to be difficult; some even refuse the treatment, or drop out before improvements are seen. The investigators have been working to develop an alternate approach which is just as effective as ERP, but which the investigators think will be much more acceptable to those who seek help for their OCD. This study will compare the traditional behavioural approach, with the newer cognitively-based approach. The investigators expect that the two treatments will both reduce the symptoms and distress of people with OCD, but that this newer cognitive therapy will have fewer people who refuse or drop out of the treatment, and will be rated as more acceptable. This research will have important implications not only for those struggling with OCD, but also for other anxiety-related problems where behavioural approaches are typically recommended, including posttraumatic stress disorder, social anxiety disorder, and other problems.