Defect engineering is an important means of improving the electrochemical performance of the Co3O4electrocatalyst in the oxygen evolution reaction (OER). In this study, operando soft X-ray absorption spectroscopy (SXAS) is used to explore the electronic structure of Co3O4under OER for the first time. The defect-rich Co3O4(D-Co3O4) has a Co2.45+state with Co2+at both octahedral (Oh) and tetrahedral (Td) sites and Co3+at Oh, whereas Co3O4has Co2.6+with Co2+and Co3+at Tdand Ohsites, respectively. SXAS reveals that upon increasing the voltage, the Co2+in D-Co3O4is converted to low-spin Co3+, some of which is further converted to low-spin Co4+most Co2+in Co3O4is converted to Co3+but rarely to Co4+. When the voltage is switched off, Co4+intermediates quickly disappear. These findings reveal Co(Oh) in D-Co3O4can be rapidly converted to active low-spin Co4+under operando conditions, which cannot be observed by ex situ XAS.