Nitrogen-doped nano-onions (NNO) were prepared as electrocatalytic materials for the oxygen reduction reaction (ORR). The nano-onions (NO), spherical graphitic material particles, were prepared by pyrolysis of nanodiamonds (ND). Oxidized NO (ONO) was prepared from NO by a modified Hummers' method, and this was mixed with urea, followed by pyrolysis, resulting in the formation of NNO. The nitrogen content and molar ratio of nitrogen-containing groups in the NNOs were varied by controlling the oxygen content of ONO to explore the effect of nitrogen content on the ORR activity. The formation of NO was confirmed by Raman spectroscopy, X-ray diffraction analysis, and high-resolution transmission electron microscopy. X-ray photoelectron spectroscopy analyses were conducted to confirm the formation of the NNO and the structures of the nitrogen-containing groups in the NNOs. The ORR activities of the NNOs were investigated using a rotating disk electrode. The NNOs showed a higher onset potential than that of NO, and the ORR activity of the NNO could be improved by increasing the number of active sites (nitrogen-containing groups) in the NNO. In addition, the NNO exhibited better long-term stability and resistance toward methanol crossover in the ORR than the platinum-based catalysts.