Background: The impacts of COVID-19 for people with obsessive-compulsive disorder (OCD) may be considerable. Online cognitive behavioural therapy (iCBT) programmes provide scalable access to psychological interventions, although the effectiveness of iCBT for OCD during COVID-19 has not been evaluated.

Aim: This study investigated the uptake and effectiveness of iCBT for OCD (both self- and clinicianguided courses) during the first 8 months of the pandemic in Australia (March to October 2020) and compared outcomes with the previous year.

Method: 1,343 adults (824/1343 (61.4%) female, mean age 33.54 years, SD = 12.00) commenced iCBT for OCD (1061 during the pandemic and 282 in the year before) and completed measures of OCD (Dimensional Obsessive-Compulsive Scale) and depression (Patient Health Questionaire-9) symptom severity, psychological distress (Kessler-10), and disability (WHO Disability Assessment Schedule) preand post-treatment.

Results: During COVID-19, there was a 522% increase in monthly course registrations compared with the previous year, with peak uptake observed between April and June 2020 (a 1191% increase compared with April to June 2019). OCD and depression symptom severity were similar for the COVID and pre-COVID groups, although COVID-19 participants were more likely to enrol in self-guided courses (versus clinicianguided). In both pre- and during-COVID groups, the OCD iCBT course was associated with medium effect size reductions in OCD (g = 0.65–0.68) and depression symptom severity (g = 0.56–0.65), medium to large reductions in psychological distress (g = 0.77–0.83) and small reductions in disability (g = 0.35–.50).

Conclusion: Results demonstrate the considerable uptake of online psychological services for those experiencing symptoms of OCD during COVID-19 and highlight the scalability of effective digital mental health services.

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.