This chapter outlines a CBT program for obsessive–compulsive disorder in patients with psychosis, focusing on cases where severe, low-insight OCD symptoms may overlap with or mimic psychotic phenomena. It begins with treatment goals and assessment using instruments such as the Yale–Brown Obsessive-Compulsive Scale, Obsessive-Compulsive Inventory-Revised, and Florida Obsessive-Compulsive Inventory. The core intervention is Exposure and Response Prevention, broken down into exposure control, in vivo and in sensu exposure, choice of gradual versus flooding strategies, and decisions about the degree of response prevention. A case vignette of a patient with schizophrenia demonstrates how ERP is implemented and how successful treatment of OCD can also improve psychotic symptoms that were partly driven by compulsive processes.