Here, we first report an octahedral Co2+-rich Co oxide with inactive Sb5+ ion as an oxygen evolution reaction (OER) electrocatalyst for efficient H2 production by lowering the cell voltage in anion exchange membrane water splitting (AEMS). To enhance the OER activity of Co-based oxides, it is crucial to increase the amount of Co4+ at OER potential, known as the fast OER active site. Using in situ X-ray absorption spectroscopy, we observed most of the octahedral Co2+ in trirutile CoSb2O6 oxidized to Co3+ before the OER potential. Furthermore, oxygen vacancies facilitated further oxidation from Co3+ to Co4+ by tuning OH– adsorption energy of Co and then produced extensive active sites for O–O bond formation in OER compared to spinel-type Co3O4. As a result, we obtained a much lower OER overpotential of 360 mV at 100 mA/cm2, providing 88% of the highest H2 energy efficiency in the AEMS by applying a cell voltage of 1.7 V.​