Manganese oxide catalysts exist in various forms of polymorphs, which have direct influence on their oxygen evolution reaction (OER) performance. However, their structure flexibility under working conditions remains a great challenge for identifying their active structures. In this work, OER performances of manganese oxide electrocatalysts with representative phase compositions (birnessite, bixbyite) were examined using operando-Raman spectroscopy. The results indicate that catalysts containing birnessite composition do not exhibit apparent OER activity before an charging activation process. Shifting of Raman peaks around similar to 640 cm(-1) can serve as an indicator for predicting their OER onset potentials. The amorphization of mixed-phase catalyst is in favor of this activation and reduces the OER onset potential by 120 mV. Moreover, the coexistence of bixbyite-like Mn(III) center and amorphized birnessite composition combines their abilities of catalyzing both electrochemical and chemical steps, creating a more bias-efficient reaction channel. This leads to the lowest Tafel slope and improved activity in the high overpotential region, suggesting that amorphous OER catalysts with structural flexibility can facilitate the OER kinetics. (C) 2018 Published by Elsevier Inc.