The answer to the question "Can electron-rich oxygen (O2-) withdraw electrons from metal centers?" is seemingly simple, but how the electron population on the M atom behaves when the O-M distance changes is a matter of controversy. A case study has been conducted for Keggin-type polyoxometalate (POM) complexes, and the first-principles electronic structure calculations were carried out not only for real POM species but also for "hypothetical" ones whose heteroatom was replaced with a point charge. From the results of natural population analysis, it was proven that even an electron-rich O2-, owing to its larger electronegativity as a neutral atom, withdraws electrons when electron redistribution occurs by the change of the bond length. In the case where O2- coexists with a cation having a large positive charge (e.g., P5+(O2-)(4) = [PO4](3-)), the gross electron population (GEP) on the M atom seemingly increases as the O atom comes closer, but this increment in GEP is not due to the role of the O atom but due to a Coulombic effect of the positive charge located on the cation. Furthermore, it was suggested that not GEP but net electron population (NEP) should be responsible for the redox properties.