Objective The comorbidity of obsessive-compulsive disorder (OCD) and personality disorders (PDs) is frequent but there are conflicting findings about which PDs are the most common. This study aimed to investigate the personality beliefs that exist on a more pathological level among OCD patients, to explore the association between personality beliefs and OCD severity, and to clarify the mediator effect of depression in this relationship. Methods 202 OCD patients and 76 healthy controls with similar sociodemographic features were included in the study. The Personality Belief Questionnaire-Short Form was administered to both groups. The Yale-Brown Obsessions and Compulsions Scale, Beck Depression Inventory, and the Beck Anxiety Inventory were administered only to the clinical sample. Results The dependent, histrionic, paranoid, borderline, and avoidant personality subscale scores were significantly higher in the OCD group than in the control group. There was an association only between OCD severity and narcissistic personality beliefs, also depression mediated the relationship between narcissistic personality and OCD severity. Conclusion Some personality beliefs at a pathological level are more common among OCD patients. Personality beliefs, as well as depression, should be routinely assessed, as they may affect OCD severity, help-seeking behavior, and response to treatment.