The well-defined low-energy features, so-called pre-edges, of oxygen K-edge X-ray absorption spectra in transition metal (TM) oxides correspond to the highly hybridized states of TM 3d and O 2p orbitals. The evolution of these features have been broadly used for studying the oxygen oxidation states in battery electrodes; however, critical questions remain on the validity of such an application and the origin of the pre-edge evolution upon cycling is yet to be clarified. Here, combined with theoretical calculations, we investigated the O-K pre-edge collected from 14 groups of electrodes and a total of 55 oxides that cover all 3d TMs used in battery cathodes with different elements, structures and electrochemical states. We show conclusive evidences that the O-K pre-edge variation is dominated by the change of TM states. More importantly, the O-K pre-edge enables a unique opportunity to project the lowest unoccupied states of all TMs onto one common energy window, which corresponds to the relative potentials of TM reduction reactions (electron filling). The summary of all XAS analysis thus delivers a universal map of the relative TM redox potentials, which reveals an unusual Cr3+/6+ redox that is further confirmed by both experiments and theory. This work provides a critical clarification on how to correctly interpret O-K pre-edge of TM oxides, which also leads to a simple but effective method to determine the relative TM redox potentials of known or unknown, usual or novel electrode materials.