OBJECTIVE: Although there is a growing number of studies on psychiatric comorbidities of obsessive-compulsive disorder (OCD), controversy remains about the strength of associations between some disorders. We present a series of systematic reviews and meta-analyses examining four psychiatric disorders frequently associated with OCD-depression, bipolar disorder, substance abuse disorder and psychosis. METHODS: Three electronic databases were searched up to May 2017; PsycINFO, Medline, and Web of Science. Meta-analyses using random effects models were conducted. RESULTS: The meta-analyses found strong associations between all comorbidities and OCD, particularly for depression. In addition, a series of systematic reviews were conducted giving information about the characteristics of these psychiatric associations. CONCLUSION: It is probable that these disorders share common etiological factors, and thus trans-diagnostic processes must be addressed when offering treatment. Clinical implications of these findings are discussed with suggestions for further research in the area.