Our work sets out to identify the operative oxygen reduction mechanism of OMIECs through a multi-faceted experimental and theoretical approach. We primarily focus our efforts on p(NDI-T2 P75), a naphthalene-1,4,5,8-tetracarboxylic-diimide-bithiophene copolymer with 75% polar side chains (Figure ). NDI-T2 copolymers are acceptor-donor copolymers and are known to have good charge transport properties relative to other n-type OMIECs. Notably, this OMIEC is stable in a broad pH range, is structurally similar to a previously described oxygen-reducing OMIEC, and has heteroatoms and functional groups reported to be vital for electrocatalysis on carbonbased materials, including N, carbonyl groups (C=O), and S. Figure . Structure of p(NDI-T2 P75), an acceptor-donor random copolymer with a backbone composed of an electron-deficient naphthalene diimide (NDI) unit and an electron-rich bithiophene (T2) unit. The hydrophilic side chains based on ethylene glycol, R , attached to 75% of monomer units (P75) facilitate solvated ion-insertion, and the hydrophobic side chains based on alkyl chains, R 2 , provide improved electrochemical stability. We combine rotating ring-disk electrochemistry, operando spectroscopy techniques, and ab initio and steady-state microkinetic simulations to evaluate the performance and mechanism of p(NDI-T2 P75) for oxygen reduction. We validate the generality of our mechanistic conclusions by benchmarking the performance of an expanded selection of p-and n-type OMIECs. The rigorous approach to studying electrochemical H2O2 production presented in this paper can be extended to materials beyond OMIECs and facilitates a guided exploration to understand the operating principles of oxygen reducing electrodes.