Background: OCD is a debilitating mental health condition. Poor access to evidencebasedpsychological treatment has motivated the implementation of low intensityinterventions in UK mental health services for OCD. Around one half of those accessingcare remain clinically unwell. Service users, providers, and treatment decision-makerscould benefit from understanding users' views of low intensity interventions and knowingwhich users are more likely to benefit from diverse low intensity approaches in order tomatch individuals with treatments of known efficacy.Aim and objectives: The primary aim of this research was to determine usercharacteristics associated with engagement and outcome in low intensity interventions forOCD. The objectives of the study were: 1) to review the published literature on predictorsof psychological therapy outcome in OCD, 2) to explore individual variation inintervention acceptability, uptake, and engagement, and 3) to identify user characteristics,which moderate intervention engagement and outcome.Methods: This mixed methods thesis was conducted in three parts. Firstly, a systematicreview of published trial data was conducted to identify predictors of therapy outcome.Secondly, primary data were collected as part of a large effectiveness trial: 1) qualitativeinterviews with 36 trial participants, randomised to one of two low intensity interventions(guided self-help; cCBT); and 2) quantitative trial assessments conducted at baseline and at3-months follow-up. Data from the systematic review and the qualitative study were usedto identify possible moderators of outcome. Confirmatory analyses were then conducted onthese variables, using data from the quantitative assessments.Results: Objective 1: The systematic review identified 43 studies, examining predictors ofoutcome. Robust effects were rare and the applied utility of these findings is limited bymethodological weaknesses. Objective 2: Six themes were identified from the qualitativestudy, three relate to the general acceptability of low intensity interventions: 1) Predisposingconcepts of high quality psychological therapies, 2) Engaging with low intensityinterventions, and 3) The perceived value of therapist support. Three are specific toindividuals with OCD: 4) Positive aspects of OCD, 5) Recognition and accommodation ofOCD users' needs, and 6) OCD disclosure. Objective 3: User attachment style, expressedemotion, OCD symptom subtype, and prior help seeking for OCD were examined inconfirmatory interaction tests. Symmetry/order/exactness symptoms were associated with agreater likelihood of engagement in guided self-help than cCBT. Contamination/washingsymptoms were associated with improved outcome in guided self-help over cCBT.Conclusions: This study has made an original contribution through using a mixed methodsdesign to identify individual differences in response to low intensity interventions in OCD.There is significant individual variation in the acceptability of, and engagement with, lowintensity interventions for OCD, linked to the mode and intensity of therapist supportpreferable to the individual user. However, few moderators could be identified. Futureresearch should focus on maximising the reliability of stratified medicine research to allowrelated findings to inform clinical decision-making.