Diamond compacts were synthesized by thermobaric processing of graphite and amorphous boron mixtures under the conditions of the diamond thermodynamic stability (at a pressure of 8–9 GPa and temperature of ∼2500 K). The boron-doped diamond-compact electrode surface was modified by its subjecting to the action of cathodic, anodic, and cathodic–anodic electrolytic plasma formed under the applying of voltage pulses with amplitude up to 300 V in Na2SO4 aqueous solution. It was found by using rotating disc electrode that the applying of sole cathodic–anodic plasma provides negligible catalytic effect with respect to the oxygen reduction reaction. However, thus pre-processed electrode acquired significant electrocatalytical activity upon the cathodic treatment, with the consequence that the reaction of O2 reduction to H2O passed predominantly by the four-electron mechanism. At the same time, the cathodic polarization of the plasma-modified electrode produced no effect on the rate constant of the electron transfer in the [Ru(NH3)6]2+/3+ redox couple; yet, the rate constant in the [Fe(CN)6]4–/3– one increased significantly. Hypothetically, the observed electrocatalytical effect in the oxygen reduction reaction is due to the formation, under the combined action of the cathodic–anodic plasma and cathodic polarization, of quinone groups at the boron-doped diamond surface; they play the role of active sites for the oxygen four-electron reduction.