By using first principles DFT calculations, we reveal oxygen reduction reaction mechanisms in N-doped graphene (N-Gr). Considering both the morphology and the concentration of dopant N atoms in bulk and edge N-Gr forms, we calculate the energies of a large number of N-Gr model systems to cover a wide range of possible N-Gr structures and determine the most stable N-Gr forms. In agreement with experiments, our DFT calculations suggest that doping levels in stable N-Gr forms are limited to less than approximately 30 at. % N, above which the hexagonal graphene framework is broken. The ground state structures of bulk and edge N-Gr forms are found to differ depending on the doping level and poisoning of the edge bonds. Oxygen reduction reaction mechanisms are evaluated by using Gibbs free-energy diagrams with and without water solvation. Our results indicate that N doping significantly alters the catalytic properties of pure graphene and that dilutely doped bulk N-Gr forms are the most active.