Several studies suggest increased prevalence-rates of obsessive-compulsive symptoms (OCS) and even of obsessive-compulsive disorder (OCD) in patients with schizophrenic disorders. Moreover, it has been recently proposed the existence of a distinct diagnostic sub-group of schizo-obsessive disorder. However, the further investigation of the OCS or OCD-schizophrenia diagnostic comorbidity presupposes the accurate clinical differential diagnosis of obsessions and compulsions from delusions and repetitive delusional behaviours, respectively. In turn, this could be facilitated by a careful comparative examination of the phenomenological features of typical obsessions/compulsions and delusions/repetitive delusional behaviours, respectively. This was precisely the primary aim of the present investigation. Our examination included seven features of obsessions/delusions (source of origin and sense of ownership of the thought, conviction, consistency with one's belief-system, awareness of its inaccuracy, awareness of its symptomatic nature, resistance, and emotional impact) and five features of repetitive behaviours (aim of repetitive behaviours, awareness of their inappropriateness, awareness of their symptomatic nature, and their immediate effect on underlying thought, and their emotional impact). Several of these clinical features, if properly and empathically investigated, can help discriminate obsessions and compulsive rituals from delusions and delusional repetitive behaviours, respectively, in patients with schizophrenic disorders. We comment on the results of our examination as well as on those of another recent similar investigation. Moreover, we also address several still controversial issues, such as the nature of insight, the diagnostic status of poor insight in OCD, the conceptualization and differential diagnosis of compulsions from other categories of repetitive behaviours, as well as the diagnostic weight assigned to compulsions in contemporary psychiatric diagnostic systems. We stress the importance of the feature of mental reflexivity for understanding the nature of insight and the ambiguous diagnostic status of poor insight in OCD which may be either a marker of the chronicity of obsessions, or a marker of their delusionality. Furthermore, we criticize two major shortcomings of contemporary psychiatric diagnostic systems (DSM-IV, DSM-V, ICD-10) in their criteria or guidelines for the diagnosis of OCD or OCS: first, the diagnostic parity between obsessions and compulsions and, second, the inadequate conceptualization of compulsions. We argue that these shortcomings might artificially inflate the clinical prevalence of OC symptoms in the course of schizophrenic disorders. Still, contrary to a recent proposal, we do not exclude on purely a priori grounds the possibility of a concurrence of genuine obsessions along with delusions in patients with schizophrenia.