The purpose of this study is to examine how feasible and acceptable it is to deliver a talking treatment called Cognitive Behavioural Therapy (CBT) in two different formats for people who have had treatment for OCD in the past but it didn't work out for them.

The two different treatment formats are CBT delivered on a weekly basis (which is approximately 12-18 hours of therapy delivered weekly for 60-90 minutes each session, followed by 1-3 monthly follow up sessions as needed) and CBT offered in an intensive format (which is having approximately 12-18 hours of therapy all in a 3-week period, followed by 1-3 monthly follow up sessions as needed).

The main aim of this study is to assess the feasibility and acceptability of Intensive CBT for treatment resistant OCD when participants are randomized to this format.

This study aims to establish an estimate of the relative efficacy of both Intensive and Weekly CBT when compared individually to wait list, for participants with a diagnosis of OCD, who have previously not responded to CBT.

Inclusion Criteria:

• Primary diagnosis of OCD (as confirmed by the administration of the Structured Clinical Interview for Diagnostic and Statistical Manual of Mental Disorders, 4th ed.) (DSM-IV) (SCID-I) .
• Participant identifies their main problem as OCD.
• Participant has not responded to one or more trials CBT
• Yale-Brown Obsessive Compulsive Scale (YBOCS) score of 16 or above.
• No change in psychotropic medication (if applicable) for at least 8 weeks prior to study entry. (If such a change has taken place there will be an option for the participant to take part once the medication has been stabilised).
• No intention to change psychotropic medication during the course of the trial
• Able to speak and read every day English

Exclusion Criteria:

• Severe psychiatric problem that requires separate treatment at an immediate basis and is linked to risk.
• Drug and/or alcohol dependence in last three months
• History of Psychosis or Bipolar Disorder