This assignment presents a comprehensive case study focusing on a six-year-old male client, Barry Simpson, diagnosed with Oppositional Defiant Disorder (ODD). The study details the client's demographic information, reasons for referral, and DSM-5 diagnosis, including diagnostic criteria, features, and severity levels. It explores contributing psychosocial and environmental factors, differential diagnoses, and an overview of the literature pertaining to the diagnosis, including prevalence in Australia, development and course, risk and prognostic factors, and gender-related factors. The case study also examines the functional consequences of ODD and proposes a psychological treatment approach. The analysis includes detailed discussions on the client's behavior, emotional state, and social interactions, providing a thorough understanding of the disorder and its implications.