Transition metals-doped noble electrocatalysts are commonly applied for the acidic oxygen evolution reaction (OER). Notwithstanding, most transition metal oxides are unstable under the conditions of acidic OER, questioning the real active sites on the surface of the electrocatalyst. Herein, RuMnOx is explored more than 30 % activity increased (at 1.7 V) after chronoamperometric activation measurement. The activities are improved when the chronoamperometric measurement potentials below 1.1 V. Coupling electrochemical measurements with Mn dissolution and Ru redeposition studies, we verify that the reconstruction of catalysts is responsible for the activated electrochemical activities. Furthermore, we demonstrate that the activity is enhanced by more hydroxyl adsorption on the surface, and the activated RuMnOx shows robust durability with a 0.027 mV/h potential increase rate for 2,600 h at current density of 10 mA cm−2.