Great effort has been contributed to exploring efficient and cost-effective oxygen reduction reaction (ORR) catalysts for fuel cell applications in the past decades. Now various electrocatalysts can be synthesized for high-performance ORR catalysis. However, the identification of the ORR active sites in many nonprecious metal-based catalysts is still difficult. This is due to the heterogeneity and complexity of the catalyst structures. For example, the active site of core–shell ORR electrocatalysts has been a continuously debatable issue, hampering the exploration of new ORR catalysts. Herein, a carbonized Co metal organic framework (Co@C) is used to uncover the ORR active sites in core–shell electrocatalysts. The surface Co particles in the Co@C sample are removed by HCl wash, and the Co cores are removed using an electrochemical activation method. The characterizations reveal that both the samples before and after the electrochemical activation show the existence of single Co species. The corresponding electrocatalysis test results indicate that neither the surface Co particles nor the encapsulated Co cores influence the ORR performance of the samples. It is deduced that the single Co species coordinated with the nitrogen in the carbon layers of the core–shell catalysts are the actual ORR active sites.