Very little is known about the comorbid presentation of obsessive-compulsive disorder (OCD) in youth with autism spectrum disorder (ASD) despite the high rates of comorbidity in clinical samples (Ivarsson and Melin, 2008; Stewart et al., 2016). This study examined the clinical expression and treatment response among children and adolescents with OCD with (n = 25) and without (n = 25) comorbid ASD (n = 50; aged 7–17 years). Youth were assessed on gold-standard measures of OCD severity and associated symptoms including functioning, mood, anxiety, comorbidity and family accommodation. The results indicated that comorbid OCD and ASD was associated with significantly higher functional impairment across school, social and home domains; significantly more externalising disorders and more comorbid disorders overall than OCD without comorbid ASD. Furthermore, families of comorbid OCD and ASD children and adolescents engaged in significantly more accommodating behaviours, and moreover, these comorbid youth had poorer treatment response at six-month follow-up. Findings from this study may be helpful in designing more targeted treatment protocols for children and adolescents with comorbid OCD and ASD.