Conducting polymers such as poly (3,4-ethylenedioxythiophene) (PEDOT) have attracted research attention as promising effective electrocatalytic materials for Oxygen Reduction Reaction (ORR). However, the complete reaction pathways leading to the hydrogen peroxide, H2O2, formation still remained unexplored. In this study, ab initio Born-Oppenheimer molecular dynamics (MD) calculations with an explicit solvent, were carried out to investigate the detailed mechanisms of the ORR on PEDOT via monitoring MD trajectories and analysing electronic energies. We confirm the outer-sphere nature of the first electron transfer and describe the essentially concerted electron-proton transfer nature of both oxygen reduction steps. Proton transfer is found to be involved in the first and second reduction steps and the role of water as an explicit solvent is outlined. Formation of the hydrogen peroxide is observed via either the reduction of hydroperoxyl radical HO2 center dot or cleavage of its peroxoadduct with PEDOT. It is therefore theoretically validated that the O2 reduction on a PEDOT may proceed a series pathway occurring simultaneously in acidic media. This approach provides an efficient and reliable means to rationalize and predict reaction mechanisms in solvent environment.