According to psychodynamic and cognitive models of obsessive-compulsive disorder (OCD), anger and aggression play an important role in the development and maintenance of the disorder. (Sub-) clinical samples with OCD have reported higher anger and anger suppression. Patients with checking-related symptoms of OCD showed a less aggressive self-concept as assessed by an Implicit Association Test (IAT). This study assessed anger and aggressiveness self-concepts in OCD as well as possible mediators of the link between OCD and aggressiveness. A total of 48 patients with OCD and 45 healthy controls were included. Measures included the State-Trait Anger Expression Inventory-II and an aggressiveness self-concept IAT (Agg-IAT). An inflated sense of responsibility, non-acceptance of emotions, and social desirability were tested as mediators. As expected, patients with OCD reported higher trait anger and anger suppression compared to healthy controls. Contrary to hypotheses, the aggressiveness self-concept (Agg-IAT) did not differ between groups. The inflated sense of responsibility mediated the relationship between group and anger suppression. Non-acceptance of negative emotions mediated the relationship between group and trait anger, as well as anger suppression. However, comorbidities and medication may account for some effect in anger suppression. Elevated trait anger and anger suppression in OCD patients could be explained by dysfunctional beliefs or maladaptive emotion regulation strategies. Emotion regulation therapy might help to enhance awareness and acceptance of emotions and possibly improve treatment outcomes.