Reducing noble-metal loading and improving the oxygen reduction reaction (ORR) performance are two of the most important topics in developing a cost-effective fuel cell system. In this study, nanocatalysts (NCs) comprising NiOx supported Pd nano-islands and a sub-nanometer Pt cluster mask (NPP) are developed. We demonstrate that the oxide structure can be suppressed along with the increase of Ni intermixed by decorating with an appropriate amount of Pt cluster mask on the surface, thus boosting the ORR performance of NPP NCs. In the optimum case, with a Pt loading of 9 wt%, the Pt decorated Ni@Pd NC showed an increase in mass activity of ∼22.74-fold (1523.7 mA mg-1) and ∼26.8-fold (671.5 mA mg-1), compared to that of the commercial Pt catalyst at 0.85 volt (vs. RHE) (67.1 mA mg-1) and 0.90 volt (vs. RHE) (24.5 mA mg-1), respectively. Such an optimization can be attributed to the proper control of the surface coverage and size of the Pt clusters. This protects the surface from oxidation, thus leaving a high density of reaction sites for oxygen adsorption in the ORR. Further increasing the Pt content results in a high density of nucleation sites and a high extent of homoatomic clustering of the Pt atoms on the NiOx@Pd surface. Such a scenario reduces the surface coverage of Pt clusters and, in this event, suppresses the ORR activity of NPP NCs. The result obtained in this study is very exciting and is an important clue for the development of next-generation ORR catalysts.