Recently, carbon nanomaterials doped with nonmetallic atoms have been used as electrocatalysts involved in oxygen reduction reactions (ORRs) because of the lack of degradation and contamination problems caused by metal dissolution, low cost, sustainability, and multifunctionality. In this study, the metal-free N-, P-, O-tridoped carbon hollow nanospheres (N, P, O-Carbon) with openings in the shell surfaces have been developed, where poly(o-phenylenediamine) hollow nanospheres with openings in the shell surfaces were chosen as a nitrogen-rich polymer, and then different phosphorus sources (such as NaH2PO2, H3PO4, and phytic acid (PA)) were introduced for heat treatment. When used as electrocatalysts, N, P, O-Carbon-PA showed the best ORR electroactivity with an onset potential (Eonset) of 0.98 V and the limit current density of 5.39 mA cm-2. The origin of high activity associated with heteroatom doping was elucidated by X-ray photoelectron spectroscopy and density functional theory. The results evidenced the high potential of N, P, O-Carbon as highly active nonmetal ORR electrocatalysts. It can be expected that the conclusions rendered herein will provide guidance for the reasonable design of other heteroatom-doped carbon for wider applications.