Acceptance and commitment therapy (ACT) has a small research base as a treatment for obsessive compulsive disorder (OCD) and related disorders. It is presumed that the process of change in ACT is an increase in psychological flexibility. This study focuses on session by session changes in psychological flexibility in 41 adults diagnosed with OCD who were treated with ACT compared with 38 individuals who received progressive relaxation training. In a randomized controlled design, participants received 8, one-hour weekly sessions with post treatment assessment one week after treatment and follow up three months later. Results showed that treatment effects were gradual with significantly better outcomes for ACT occurring in the final two sessions. Multiple levels of analyses show that changes in psychological flexibility predict changes in OCD better than changes in OCD severity predicting changes in psychological flexibility. Similarly, multiple levels of mediational analyses showed that post treatment levels of psychological flexibility mediate pretreatment to follow up reductions in OCD severity. © 2015 Association for Contextual Behavioral Science.