Obsessive-compulsive disorder (OCD) and autism spectrum disorder (ASD) frequently co-occur. Standard cognitive behaviour therapy (CBT) for OCD outcomes are poorer in young people with ASD, compared to those without. The aim of this naturalistic study was to evaluate the effectiveness of a novel adolescent autism-adapted CBT manual for OCD in a specialist clinical setting. Additionally, we examined whether treatment gains were maintained at 3-month follow-up. Thirty-four adolescents underwent CBT; at the end of treatment, 51.51% were treatment responders and 21.21% were in remission. At 3-month follow-up, 52.94% were responders and 35.29% remitters. Significant improvements were also observed on a range of secondary measures, including family accommodation and global functioning. This study indicates this adapted package of CBT is associated with significant improvements in OCD outcomes, with superior outcomes to those reported in previous studies. Further investigation of the generalizability of these results, as well as dissemination to different settings, is warranted.