First-principles DFT computations are performed to explain the origin and the mechanism of oxygen reduction reaction (ORR) on Co-N(x) (x = 2, 4) based self-assembled carbon supported electrocatalysts in alkaline and acidic media. The results show that the formation of graphitic Co-N(4) defect is energetically more favorable than the formation of graphitic Co-N(2) defect. Furthermore graphitic Co-N(4) defects are predicted to be stable at all potentials (U = 0-1.23 V) in the present study while Co-N(2) defects are predicted to be unstable at high potentials. Therefore the Co-N(4) defect is predicted to be the dominant in-plane graphitic defect in Co-N(x)/C electrocatalysts. O(2) chemisorbs to Co-N(4) and Co-N(2) defects indicating that both defect motifs are active for the reduction of O(2) to peroxide. However, the weak interaction between peroxide and Co-N(4) defect shows that this defect does not promote complete ORR and a second site for the reduction of peroxide is required, supporting a 2 × 2e(-) dual site ORR mechanism independent of pH of the electrolyte. In contrast, the much stronger interaction between peroxide and Co-N(2) defect supports a 2 × 2e(-) single site ORR mechanism in alkaline and acidic media.