This case report is about a 31-year-old married female with a variety of obsessions, primarily focusing on harming obsessions. Because of anxiety, numerous washing rituals have taken place as well as the avoidance of situations. This client was diagnosed with Obsessive-Compulsive Disorder (OCD) and was seen for 25 sessions of individual cognitive therapy. The aim of this study therefore was to illustrate the assessment, case conceptualization, and course of treatment in a severe obsessive-compulsive client, using cognitive therapy. Several specific cognitive interventions, which address the estimations of catastrophes and the perceptions of personal responsibility, are presented and illustrated. At posttreatment and at follow-up, a clinically significant change for improvement was demonstrated for the obsessive-compulsive symptoms.