The lack of low-cost and efficient oxygen evolution reaction (OER) catalysts in acid media has significantly limited the development of proton exchange membrane water electrolysis, which can perfectly integrate with intermittent renewable energy. Recently, iridium and non-noble metal mixed oxides have been identified as promising candidates for the OER in acid media. Herein, we report a novel double mixed metal oxide BaIrO_2.937/La₃IrO₇ with a surface of IrO_x formed by Ba and La leaching as an efficient electrocatalyst for boosting the sluggish OER. In 0.1 M HClO₄, a low overpotential of 290 mV is required for BaIrO_2.937/La₃IrO₇ to achieve the benchmark of 10 mA cm⁻², comparable to commercial IrO₂ and most reported acidic OER electrocatalysts. Moreover, a smaller Tafel slope of BaIrO_2.937/La₃IrO₇ signifies fast reaction kinetics. Significantly important is that the iridium based mass activity was 561.6 A g_Ir⁻¹ at 1.63 V vs. a reversible hydrogen electrode, which is 3.9 times as high as that of commercial IrO₂. This work highlights the smart design of double mixed metal oxides with surface reconstruction as highly active and low cost electrocatalysts for the OER in acidic media.