Metal oxides show great potential in catalyzing the oxygen evolution reaction (OER), which is taken as the bottleneck of many energy-conversion and -storage processes. However, it is still a major challenge to deeply understand the catalytic mechanism and to rapidly screen out novel metal-oxide catalysts. Herein, we find that the trend of adsorption energies of O-intermediates and the theoretical overpotentials on metal monoxides (MO), metal dioxides (MO2), and perovskite oxides (ABO3) can be determined using the descriptor ψ, which is related to the valence-electron numbers and the electronegativities of the active center. The underlying mechanism is that ψ reflects the p-band properties of superficial and adsorbed O atoms of metal oxides and thus can reveal the commonality and difference of adsorption and catalysis of the three types of metal oxides. Moreover, the ψ-determined relationships indicate the possibility of breaking the previously proposed thermodynamic limitation on the post-transition metal oxides and rationalizing the trend of experimental OER catalytic activity of metal oxide catalysts. With this easily accessible intrinsic descriptor ψ, we provide a convenient and feasible scheme for designing new candidates for OER catalysts.