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Onion-like carbon (OLC) is known to form as an intermediate structure when nanodiamond (ND) is heat treated at 1100–1500 °C. In this letter, we report on the electrochemical activity of its oxygen reduction reaction (ORR) when prepared at temperatures from 1000 to 2000 °C. The ORR activity is found to exhibit excellent correlation with the degree of defects, with OLC prepared at 1400 °C found to have the highest activity. This is attributed to the formation of curved graphitic layers containing edges during ND-OLC transformation, which function as active sites for the adsorption and subsequent reduction of oxygen.