Background: Obsessive–compulsive disorder (OCD) is a relatively common and disabling condition. Objectives: To determine the clinical effectiveness, acceptability and cost-effectiveness of pharmacological and psychological interventions for the treatment of OCD in children, adolescents and adults. Data sources: We searched the Cochrane Collaboration Depression, Anxiety and Neurosis Trials Registers, which includes trials from routine searches of all the major databases. Searches were conducted from inception to 31 December 2014. Review methods: We undertook a systematic review and network meta-analysis (NMA) of the clinical effectiveness and acceptability of available treatments. Outcomes for effectiveness included mean differences in the total scores of the Yale–Brown Obsessive–Compulsive Scale or its children’s version and total dropouts for acceptability. For the cost-effectiveness analysis, we developed a probabilistic model informed by the results of the NMA. All analyses were performed using OpenBUGS version 3.2.3 (members of OpenBUGS Project Management Group; see www.openbugs.net). Results: We included 86 randomised controlled trials (RCTs) in our systematic review. In the NMA we included 71 RCTs (54 in adults and 17 in children and adolescents) for effectiveness and 71 for acceptability (53 in adults and 18 in children and adolescents), comprising 7643 and 7942 randomised patients available for analysis, respectively. In general, the studies were of medium quality. The results of the NMA showed that in adults all selective serotonin reuptake inhibitors (SSRIs) and clomipramine had greater effects than drug placebo. There were no differences between SSRIs, and a trend for clomipramine to be more effective did not reach statistical significance. All active psychological therapies had greater effects than drug placebo. Behavioural therapy (BT) and cognitive therapy (CT) had greater effects than psychological placebo, but cognitive–behavioural therapy (CBT) did not. BT and CT, but not CBT, had greater effects than medications, but there are considerable uncertainty and methodological limitations that should be taken into account. In children and adolescents, CBT and BT had greater effects than drug placebo, but differences compared with psychological placebo did not reach statistical significance. SSRIs as a class showed a trend for superiority over drug placebo, but the difference did not reach statistical significance. However, the superiority of some individual drugs (fluoxetine, sertraline) was marginally statistically significant. Regarding acceptability, all interventions except clomipramine had good tolerability. In adults, CT and BT had the highest probability of being most cost-effective at conventional National Institute for Health and Care Excellence thresholds. In children and adolescents, CBT or CBT combined with a SSRI were more likely to be cost-effective. The results are uncertain and sensitive to assumptions about treatment effect and the exclusion of trials at high risk of bias. Limitations: The majority of psychological trials included patients who were taking medications. There were few studies in children and adolescents. Conclusions: In adults, psychological interventions, clomipramine, SSRIs or combinations of these are all effective, whereas in children and adolescents, psychological interventions, either as monotherapy or combined with specific SSRIs, were more likely to be effective. Future RCTs should improve their design, in particular for psychotherapy or combined interventions.