Background: The Obsessive-Compulsive Treatment Efficacy randomised controlled Trial emerged from a research recommendation in National Institute for Health and Care Excellence obsessive-compulsive disorder (OCD) guidelines, which specified the need to evaluate cognitive-behavioural therapy (CBT) treatment intensity formats. Objectives: To determine the clinical effectiveness and cost-effectiveness of two low-intensity CBT interventions [supported computerised cognitive-behavioural therapy (cCBT) and guided self-help]: (1) compared with waiting list for high-intensity CBT in adults with OCD at 3 months; and (2) plus high-intensity CBT compared with waiting list plus high-intensity CBT in adults with OCD at 12 months. To determine patient and professional acceptability of low-intensity CBT interventions. Design: A three-arm, multicentre, randomised controlled trial. Setting: Improving Access to Psychological Therapies services and primary/secondary care mental health services in 15 NHS trusts. Participants: Patients aged ≥ 18 years meeting Diagnostic and Statistical Manual of Mental Disorders-Fourth Edition criteria for OCD, on a waiting list for high-intensity CBT and scoring ≥ 16 on the Yale-Brown Obsessive Compulsive Scale Interventions: Participants were randomised to (1) supported cCBT, (2) guided self-help or (3) a waiting list for high-intensity CBT. Main outcome measures: The primary outcome was OCD symptoms using the Yale-Brown Obsessive Compulsive Scale - Observer Rated. Results: Patients were recruited from 14 NHS trusts between February 2011 and May 2014. Follow-up data collection was complete by May 2015. There were 475 patients randomised: supported cCBT (n = 158); guided self-help (n = 158) and waiting list for high-intensity CBT (n = 159). Two patients were excluded post randomisation (one supported cCBT and one waiting list for high-intensity CBT); therefore, data were analysed for 473 patients. In the short term, prior to accessing high-intensity CBT, guided self-help demonstrated statistically significant benefits over waiting list, but these benefits did not meet the prespecified criterion for clinical significance [adjusted mean difference -1.91, 95% confidence interval (CI) -3.27 to -0.55; p = 0.006]. Supported cCBT did not demonstrate any significant benefit (adjusted mean difference -0.71, 95% CI -2.12 to 0.70). In the longer term, access to guided self-help and supported cCBT, prior to high-intensity CBT, did not lead to differences in outcomes compared with access to highintensity CBT alone. Access to guided self-help and supported cCBT led to significant reductions in the uptake of high-intensity CBT; this did not seem to compromise patient outcomes at 12 months. Taking a decision-making approach, which focuses on which decision has a higher probability of being cost-effective, rather than the statistical significance of the results, there was little evidence that supported cCBT and guided self-help are cost-effective at the 3-month follow-up compared with a waiting list. However, by the 12-month follow-up, data suggested a greater probability of guided self-help being cost-effective than a waiting list from the health- and social-care perspective (60%) and the societal perspective (80%), and of supported cCBT being cost-effective compared with a waiting list from both perspectives (70%). Qualitative interviews found that guided self-help was more acceptable to patients than supported cCBT. Professionals acknowledged the advantages of low intensity interventions at a population level. No adverse events occurred during the trial that were deemed to be suspected or unexpected serious events. Limitations: A significant issue in the interpretation of the results concerns the high level of access to high-intensity CBT during the waiting list period. Conclusions: Although low-intensity interventions are not associated with clinically significant improvements in OCD symptoms, economic analysis over 12 months suggests that low-intensity interventions are cost-effective and may have an important role in OCD care pathways. Further research to enhance the clinical effectiveness of these interventions may be warranted, alongside research on how best to incorporate them into care pathways.