Objective To reliably assess biomarkers for treatment response and evaluate quantitative electroencephalography (qEEG) changes in treatment resistant obsessive-compulsive disorder (OCD) after deep transcranial magnetic stimulation (dTMS). Method Treatment-resistant obsessive-compulsive disorder sixty patients (35 F, 25 M) after dTMS were separated into two groups based on Yale-Brown Obsessive-Compulsive Scale (Y-BOCS) score decrease by 35% or more (Treatment-responsive (TRP)) and less than 35% (Treatment-resistant (TRS)). Quantitative electroencephalography (QEEG) recordings were compared between the TRP and TRS groups using the Mann-Whitney U test, and the pre- and post-treatment periods were analyzed using the Wilcoxon Signed Rank Test. Binary logistic regression analysis was done for the prediction of treatment response. QEEG changes. Results TRS (9 F, 11 M; age (year) = 37.1 +- 14.7) and TRP (26 F, 14 M; age (year) = 31.2 +- 11.7) groups were similar in terms of medication, sex, age, duration of illness, baseline YBOCS and all four baseline qEEG power bands and alpha asymmetries. Patients with OCD who show higher delta F3 and lower age were more likely to respond to dTMS and pharmacological treatment. Following neurostimulation and pharmacological intervention, parietal and temporal theta, and parietal delta power bands were significantly higher in the TRP group than TRS group. Central and temporal theta, temporal alpha and beta power bands were significantly increased after treatment when paired with pre-treatment. Conclusion Higher dorsolateral lateral delta power and lower age were more likely to respond to dTMS. Slow oscillations bands change was prominent for the dTMS responsiveness in treatment resistant OCD.