Oxygen reduction reaction (ORR) is investigated on bulk PdO-based catalysts (oxides of Pd and Pd₃Co) in oxygen-saturated 0.1 M HClO₄ to establish the role of surface oxides and adsorbed hydrogen in the activity and product selectivity (H₂O/H₂O₂). The initial voltammetric features suggest that the oxides are inactive toward ORR. The evolution of the ORR voltammograms and potential-dependent H₂O₂ generation features on the PdO catalyst suggest gradual and parallel in situ reduction of the bulk PdO phase below ∼0.4 V in the hydrogen underpotential deposition (H_upd) region; the reduction of the bulk PdO catalyst is confirmed from the X-ray photoelectron spectra (XPS) and X-ray diffraction (XRD) patterns. The potential-dependent H₂O₂ generation features originate due to the presence of surface oxides and adsorbed hydrogen; this is further confirmed using halide ions (Cl⁻ and Br⁻) and peroxide as the external impurities.