Spinel Co3O4, comprising two types of cobalt ions: one Co2+ in the tetrahedral site (Co2+ Td) and the other two Co3+ in the octahedral site (Co3+ Oh), has been widely explored as a promising oxygen evolution reaction (OER) catalyst for water electrolysis. However, the roles of two geometrical cobalt ions toward the OER have remained elusive. Here, we investigated the geometrical-site-dependent OER activity of Co3O4 catalyst by substituting Co2+ Td and Co3+ Oh with inactive Zn2+ and Al3+, respectively. Following a thorough in operando analysis by electrochemical impedance spectroscopy and X-ray absorption spectroscopy, it was revealed that Co2+ Td site is responsible for the formation of cobalt oxyhydroxide (CoOOH), which acted as the active site for water oxidation.