Despite significant advances in the study of obsessive-compulsive disorder (OCD), important questions remain about the disorder's public health significance, appropriate diagnostic classification, and clinical heterogeneity. These issues were explored using data from the National Comorbidity Survey Replication, a nationally representative survey of US adults. A subsample of 2073 respondents was assessed for lifetime Diagnostic and Statistical Manual of Mental Disorders, 4th edn (DSM-IV) OCD. More than one quarter of respondents reported experiencing obsessions or compulsions at some time in their lives. While conditional probability of OCD was strongly associated with the number of obsessions and compulsions reported, only small proportions of respondents met full DSM-IV criteria for lifetime (2.3%) or 12-month (1.2%) OCD. OCD is associated with substantial comorbidity, not only with anxiety and mood disorders but also with impulse-control and substance use disorders. Severity of OCD, assessed by an adapted version of the Yale-Brown Obsessive Compulsive Scale, is associated with poor insight, high comorbidity, high role impairment, and high probability of seeking treatment. The high prevalence of subthreshold OCD symptoms may help explain past inconsistencies in prevalence estimates across surveys and suggests that the public health burden of OCD may be greater than its low prevalence implies. Evidence of a preponderance of early onset cases in men, high comorbidity with a wide range of disorders, and reliable associations between disorder severity and key outcomes may have implications for how OCD is classified in DSM-V.