The integration of n-type (electron-transporting) polymers with oxidase enzymes has allowed building high-performance organic electrochemical transistor (OECT) based metabolite sensors. Yet, the operation mechanism of these devices is poorly understood. Here, the process is investigated for the conversion of metabolite oxidation to electrical current in an n-type organic electrochemical transistor (n-OECT). By monitoring oxygen (O₂), hydrogen peroxide, and pH changes in the electrolyte as well as the potential of each electrical contact of the n-OECT during glucose detection, light is shed on the physical phenomena occurring at the polymer-enzyme interface. It is shown that the n-type film performs O₂ reduction reaction in its doped state and that the n-OECT characteristics are sensitive to O₂. A correlation is found between the consumption of electrolyte-dissolved O₂ and the generation of n-OECT current during the metabolite oxidation. The results demonstrate how the sensitivity of a polymer to O₂, species known to deteriorate the performance of many semiconductor devices, becomes a feature to exploit in sensor applications. The importance of in operando analysis of the electrolyte composition and the terminal potentials is highlighted for understanding the operation mechanism of bioelectronic devices and for sensor design and materials development.