A growing evidence base supports the efficacy of Internet-based cognitive behaviour therapy (iCBT) for obsessive compulsive disorder (OCD). However, very few studies have evaluated the effectiveness of iCBT for OCD in routine care settings which service the bulk of a population's health needs. This study evaluated the treatment outcomes of 309 adults who enrolled in an online CBT course for their symptoms of OCD. Most patients (62.8%) enrolled in a self-help or unguided iCBT course, and the remainder were supported by a routine care, community clinician. The entire iCBT course was completed by 39.3% of participants, with 61.2% completing the majority of iCBT lessons. Higher baseline depression symptom severity predicted poorer adherence, while the presence of a supervising clinician was associated with greater adherence. Medium effect size reductions in OCD symptom severity (g= .61), depression symptom severity (g= .56), and psychological distress (g= .69) were observed from pre-to post-treatment. Pre-treatment OCD and depression symptom severity predicted post-treatment OCD symptom severity. Current results support the use of iCBT for OCD in routine care settings, and as health care resources are finite, iCBT for OCD may serve as a useful, low-intensity, and scalable addition to the existing healthcare infrastructure.