Transition‐metal oxides as electrocatalysts for the oxygen evolution reaction (OER) provide a promising route to face the energy and environmental crisis issues. Although palmeirite oxide A2Mo3O8 as OER catalyst has been explored, the correlation between its active sites (tetrahedral or octahedral) and OER performance has been elusive. Now, magnetic Co2Mo3O8@NC‐800 composed of highly crystallized Co2Mo3O8 nanosheets and ultrathin N‐rich carbon layer is shown to be an efficient OER catalyst. The catalyst exhibits favorable performance with an overpotential of 331 mV@10 mA cm−2 and 422 mV@40 mA cm−2, and a full water‐splitting electrolyzer with it as anode catalyst shows a cell voltage of 1.67 V@10 mA cm−2 in alkaline. Combined HAADFSTEM, magnetic, and computational results show that factors influencing the OER performance can be attributed to the tetrahedral Co sites (high spin, t23e4), which improve the OER kinetics of rate‐determining step to form *OOH. Magnetic Co2Mo3O8@NC‐800 composed of single‐crystal Co2Mo3O8 and ultrathin nitrogen‐rich carbon was synthesized to uncover its OER active sites (Td Co2+ or Oh Co2+). Electrochemical data, magnetism data, and computations suggest that the Td Co2+ atoms (high spin, t23e4) in Co2Mo3O8 act as active sites facilitating the rate‐determining step, forming *OOH to promote the reaction kinetics for OER.