The oxygen reduction reaction (ORR) was studied on CO-treated and untreated (111)-(100) nanofaceted platinum surfaces [Komanicky et al. J. Phys. Chem. 2005, 109, 23543] in sulfuric and perchloric acids using the rotating disk electrode technique. Activities of nanofaceted surfaces are found to be considerably higher than a simple average of the activities of (111) and (100) surfaces. We find that the high activity in sulfuric acid is consistent with the higher activity of (111) facets. It is due the weaker sulfate adsorption on finite-size (111) surfaces than on (111) single crystal surfaces where the ORR activity is suppressed by strong sulfate adsorption. However, the high activity found in the weakly absorbing perchloric acid cannot be explained by the finite-size effect, since the activities are reportedly insensitive to terrace sizes [Macia, M. D.; et al. J. Electroanal. Chem., 2004, 564, 141]. We propose a cooperative activity, unique to nanoscale objects, which results from oxy species crossing over between adjacent facets in nanometer proximities.