The development of acid-stable oxygen evolution reaction electrocatalysts is essential for high-performance water splitting. Here, we report an electrocatalyst with Ru-atom-array patches supported on α-MnO 2 (Ru/MnO 2 ) for the oxygen evolution reaction following a mechanism that involves only *O and *OH species as intermediates. This mechanism allows direct O–O radical coupling for O 2 evolution. Ru/MnO 2 shows high activity (161 mV at 10 mA cm −2 ) and outstanding stability with small degradation after 200 h operation, making it one of the best-performing acid-stable oxygen evolution reaction catalysts. Operando vibrational and mass spectroscopy measurements were performed to probe the reaction intermediates and gaseous products for validating the oxygen evolution reaction pathway. First-principles calculations confirmed the cooperative catalysis mechanism with a reduced energy barrier. Time-dependent elemental analysis demonstrated the occurrence of the in-situ dynamic cation exchange reaction during the oxygen evolution reaction, which is the key for triggering the reconstruction of Ru atoms into the ordered array with high durability.