We study the characteristics of oxygen adsorption on metal-free carbon-based cathode catalysts derived from nitrogen-containing polyamide (PA) and nitrogen-free phenolic resin (PhRs). Electrochemical analysis and Raman spectroscopy showed higher 2-electron oxygen reduction reaction (ORR) activity and more defect sites in PA than PhRs. The increase in the amount of adsorbed oxygen in PA was also identified by oxygen adsorption isotherms. In situ X-ray photoelectron spectroscopy revealed that graphite-like nitrogen contributes to oxygen adsorption and CO components are dominant in PA. These experimental results indicate that the adsorbed CO components near the graphite-like nitrogen can be assigned as active sites for 2-electron ORR.