Background: Obsessive compulsive disorder is a distressing psychiatric illness with considerable treatment resistance rates. Prediction of treatment response leads to an increase in patient compliance and a decrease in morbidity. To decrease the treatment resistance rates, valid and useful instruments have been searched. Quantitative electroencephalography (QEEG) based markers have been objective predictors of the treatment response in psychiatric disorders. Aim: This retrospective pilot study aims to explore QEEG as a biomarker to predict early response to paroxetine in OCD patients. Method: Resting state QEEG and Yale-Brown Obsessive Compulsive Scale (Y-BOCS) were administered to 30 drug-free OCD patients without comorbidity. After maximum 12-week of treatment with paroxetine, patients with and without an early improvement were classified based on at least a 35% reduction in Y-BOCS scores. Pretreatment QEEG data were compared between the two groups. Results: Pre-treatment gamma, gamma 1 and gamma 2 oscillations were significantly higher in OCD patients who did not show an early improvement. Conclusion: These preliminary results indicate that gamma oscillations could be acknowledged as the electrophysiological predictors of early clinical outcomes of OCD patients during paroxetine treatment.