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 O-2 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. (C) 2018 Elsevier Inc. All rights reserved.