We report on an investigation of the overall mechanism of the reaction of reduction of molecular oxygen (ORR reaction) on poly-terthiophene by means of Density Functional Theory (DFT) computation. The calculations predict a large electron affinity of terthiophene (THP) of ∼2.4 eV in aqueous phase that supports the idea that activation of the polymer catalyst occurs via an initial one-electron reduction step. The activation step is followed by O2 addition and a one-proton-one-electron coupled reduction to form a key stable peroxide radical anion intermediate THP-OOH-. The anion undergoes a unimolecular dissociation leading to hydro-peroxide OOH- anion formation (the 2-electron reduction pathway of molecular oxygen) over an activation barrier of ∼ 0.16 eV selectively over hydroxide OH- anion (the 4-electron reduction pathway of O2) over an activation barrier of ∼ 0.29 eV. Henry Eyring's transition state theory predicts the rate of OOH- evolution to be ∼ two orders of magnitude faster than OH- evolution.