In this study, we systematically investigated NH4+ oxidation at the bioanode with different O2 and NH4+-N ratios (O₂/NH₄⁺-N) of 0.5, 1.0 and 1.5. Our specific focus was on achieving high NH4+ oxidation while simultaneously maintaining the selectivity of NH4+ oxidation to N2. Additionally, the diversity of N-related functional bacteria was characterized to elucidate the underlying N conversion reactions in the MEC systems. This study elucidates the influence of microoxic levels of O2 on NH4+ oxidation at bioanodes and providing insights for process optimization.