The differential diagnosis of obsessive–compulsive disorder (OCD) and schizophrenia-spectrum disorders can be difficult. In the current diagnostic criteria, basic concepts such as obsession and delusion overlap. This study examined lifetime schizophrenia-spectrum psychopathology, including subtle schizotypal symptomatology and subjective anomalies such as self-disorders, in a sample diagnosed with OCD in a specialized setting. The study also examined the differential diagnostic potential of the classic psychopathological notions of true obsession (‘with resistance’) and pseudo-obsession. The study involved 42 outpatients diagnosed with OCD at two clinics specialized in the treatment of OCD. The patients underwent semi-structured, narrative interviews assessing a comprehensive battery of psychopathological instruments. The final lifetime research-diagnosis was based on a consensus between a senior clinical psychiatrist and an experienced research clinician. The study found that 29% of the patients fulfilled criteria of schizophrenia or another non-affective psychosis as main, lifetime DSM-5 research-diagnosis. Another 33% received a research-diagnosis of schizotypal personality disorder, 10% a research-diagnosis of major depression and 29% a main research-diagnosis of OCD. Self-disorders aggregated in the schizophrenia-spectrum groups. True obsessions had a specificity of 93% and a sensitivity of 58% for a main diagnosis of OCD. In conclusion, a high proportion of clinically diagnosed OCD patients fulfilled diagnostic criteria of a schizophrenia-spectrum disorder. The conspicuous obsessive–compulsive symptomatology may have resulted in a disregard of psychotic symptoms and other psychopathology. Furthermore, the differentiation of obsessions from related psychopathological phenomena is insufficient and a conceptual and empirical effort in this domain is required in the future.