Various psychological, social, genetic and biochemical factors are thought to be involved in etiology of obsessive–compulsive disorder (OCD). To the best of our knowledge there are no studies investigating the effects of free radicals in children and adolescents with OCD. This study evaluated total oxidant and antioxidant status, oxidative stress index, and arylesterase and paraoxonase activity in children and adolescents with OCD. The study included 28 patients diagnosed with OCD and 36 healthy children as an age- and sex-matched control group. Their serum total oxidant status (TOS) and total antioxidant status (TAS) were measured and the oxidative stress index (OSI) was calculated. Although serum TOS and OSI values in the OCD patients were significantly higher than those in the control group (p = 0.008, p < 0.001, respectively), TAS and paraxonase activity were significantly lower ( p < 0.001 for both). However, no statistically significant difference in arylesterase activity was found (p > 0.05). The increase in oxidative status and decrease in antioxidants in patients with OCD demonstrate that oxidative stress may have an important role in the pathophysiology of the disease. It has been suggested that drugs that contain antioxidants should be added to conventional pharmacotherapy during follow-up.