Various studies reported remarkably high prevalence rates of obsessive-compulsive symptoms (OCS) in patients with a psychotic disorder. Little is known about the pathogenesis of this co-occurrence. The current study aimed to investigate the contribution of shared underlying risk factors, such as childhood trauma and neuroticism, to the onset and course of OCS in patients with psychosis. Data were retrieved from 161 patients with psychosis included in the 'Genetic Risk and Outcome in Psychosis' project. Patients completed measures of OCS and psychotic symptoms at study entrance and three years later. Additionally, childhood maltreatment and neuroticism were assessed. Between-group comparisons revealed increased neuroticism and positive symptoms in patients who reported comorbid OCS compared to OCS-free patients. Subsequent mediation analyses suggested a small effect of childhood abuse on comorbid OCS severity at baseline, which was mediated by positive symptom severity. Additionally, results showed a mediating effect of neuroticism as well as a moderating effect of positive symptoms on the course of OCS severity over time. OCS severity in patients with psychosis might thus be associated with common vulnerability factors, such as childhood abuse and neuroticism. Furthermore, the severity of positive symptoms might be associated with more severe or persistent comorbid OCS.