The surface-reconstruction-induced layer on transition metal-based catalysts during electrocatalytic oxygen evolution reaction (OER) is considered to be responsible for their catalytic activity. However, how the active layer forms and works in the OER process has not been understood clearly. Here, we demonstrate a facile approach to reveal the mechanism for the formation of active layer and its role in OER by fabricating the NiCo-based hybrid as a precatalyst. We find that the anodization can improve its OER activity, leading to a low OER overpotential of 276 mV at 20 mA cm and favorable durability (≥100 h). In-situ Raman spectroscopy and ex-situ characterizations reveal that the Ni-CoOOH nanosheets array with promising surface wettability is generated via deep reconstruction during the OER process and the high-valence Co is the active site for efficient oxygen evolution. Theoretical analysis further illustrates that the Co site can be activated by the Ni incorporation, thus optimizing the adsorption capability toward the intermediates.