© 2018 American Chemical Society. A good understanding of the correlation between electronic properties and catalytic performance is vital to the rational design of active oxygen evolution reaction (OER) catalysts. Here, a volcano-shaped correlation between the OER activity and the Cr substitution amount was found for spinel oxides ZnFe2-xCrxO4(x = 0-2), in which Zn2+resides in tetrahedral (Td) sites while Fe3+and Cr3+occupy octahedral (Oh) sites. Such a relationship probably is because Cr substitution tunes the egoccupancy of Oh-site transition metals (TMOh) via the Oh-Oh superexchange effect. Density functional calculations further revealed the hybridization degree between the TMOh3d and the O 2p states, and a volcano-shaped trend was also found in the variation of TMOh3d-O 2p hybridization with the amount of Cr substitution. The good correlation between the OER activity and the hybridization highlights the important role of metal-oxygen hybridization in determining the OER activity of these spinel oxides.