The electrocatalytic activity of bulk Au and Au nanoparticles (AuNPs) toward the oxygen reduction reaction (ORR), before and after the electrochemical pretreatment, was investigated in a 0.05 M phosphate buffer solution (pH = 7.4). Both the Au and AuNPs were pretreated by repetitive potential cycling between -0.2 and +1.0 V (vs SCE). Rotating disk electrode (RDE) voltammetric studies showed that a more favorable ORR occurred at the AuNPs than at the Au. Interestingly, increased number of electrons (n) transferred in the ORR (∼4) and more positive ORR onset potential were observed at both the Au and AuNPs after the pretreatment. In particular, for the bulk Au, a simple electrochemical pretreatment produced remarkably improved ORR activity, i.e., a considerable positive shift in the ORR onset potential by ∼300 mV as well as a significant increase in the n number (from <2 to ∼4). The greatest catalytic activity was observed at the pretreated AuNPs. The enhanced catalytic activity by the electrochemical pretreatment was attributed to the formation of thick Au oxide layers, which was also demonstrated with a thermally pretreated Au microwire electrode. The pretreated Au and AuNPs also showed higher activity toward the oxidation of hydrogen peroxide than the corresponding untreated ones.