CBT for obsessive-compulsive disorder (OCD) is a strong challenge to the contention that common factors explain most of the variance in outcomes in all therapies and all disorders, given that the treatment is focused and placebo response is low. In this study, the relative contributions of expectancy and therapeutic alliance as predictors of outcome in the treatment of OCD are examined and compared to the contribution of specific treatment effects. One hundred and eight patients with OCD were randomly assigned to two forms of CBT: exposure and response prevention (EX/RP) or stress management training (SMT). Measures of OCD symptoms, quality of life, therapist and patient expectancy and alliance were collected at several timepoints. Treatment type was a substantially stronger predictor of symptom reduction compared to alliance and expectancy. However, neither specific nor common factors predicted improvement in quality of life very well. Only in EX/RP, symptom change was associated with subsequent changes in alliance. Finally, therapist effects were estimated using Bayesian methods and were negligible. In the context of CBT for OCD, the data support the specific factor model, and suggest that the relative contribution of common vs. specific factors likely varies by disorder and by treatment type.