Here, we report an efficient surface decorated, e.g. oxidation and nitrogen doped, cobalt sulfide (O-N-Co9S8) oxygen electrocatalyst, which shows excellent activity especially for oxygen evolution reaction (OER), and good stability over 900 charge-discharge cycles at 10 mA cm−2 in Zinc-air battery. Moreover, we found that O-N-Co9S8 was completely converted into Co3O4 after OER, showing oxide is actual active phase. Density functional theory calculations reveal the continuous exposure of oxidized surface Co sites during O-N-Co9S8 → Co3O4 is essential for its high OER activity. These Co sites promote the kinetics for OH∗ transformation to O∗ and also ensure fast O2 desorption. Once Co3O4 is generated, the high activity is contributed by its resulting characteristic surfaces. Thus, we propose and demonstrate that oxides in-situ generated during OER are more active than the directly calcined oxides. This work advances fundamental insight of metal chalcogenides “catalysts” and guides the design of active OER catalysts.