Objective <p>Preliminary clinical research supports the potential effectiveness of mindfulness-based interventions for obsessive-compulsive disorder (OCD). The current naturalistic community treatment study aimed to assess the comparative effectiveness of mindfulness-based cognitive therapy (MBCT) and cognitive behavioural therapy (CBT) and to explore whether the hypothesized treatment mechanisms in CBT and MBCT relating to improved clinical outcomes are distinct or overlap.</p> Methods <p>Clinical participants (<i>N</i> = 245) in a university-affiliated specialty OCD assessment and treatment centre, with primary DSM-5 diagnosed OCD experiencing clinically significant obsessive-compulsive symptoms completed structured group interventions for CBT or MBCT and reported symptom, cognitive, and mindfulness ratings across treatment.</p> Results <p>The results indicated that both CBT (<i>d</i> = 1.64) and MBCT (<i>d</i> = 1.34) treatments produced clinically and statistically large and equivalent effects on the primary measure of obsessive-compulsive symptom severity. In the examination of putative treatment mechanisms, model fit was acceptable, and results supported the hypothesized cross-lagged associations between change in obsessive beliefs and symptom reduction in CBT, and cross-lagged associations between change in self-compassion and symptom reduction in MBCT. </p> Conclusions <p>These results provide preliminary evidence for the effectiveness of mindfulness-based interventions in OCD and point to the possibility of specific treatment mechanisms distinct from CBT, although this will require further examination in randomized controlled trials.</p>