Organic electrochemical transistors (OECTs) combine electron/ionic transport with organic semiconductor flexibility to connect biology and electronics. As they approach industrial use, optimizing performance requires accurate modeling of their structure. This study presents a two-dimensional (2D) OECT model based on Nernst–Planck–Poisson equations that explicitly includes volumetric capacitance (C<inf>V</inf>). Unlike previous models that ignore C<inf>V</inf>, our model highlights its essential role in OECT operation, allowing us to accurately match the measured output currents of PEDOT:PSS printed OECTs. We studied how parameters like diffusion coefficients of holes and ions, fixed anion concentration, and intrinsic capacitance affect transistor performance. We analyze existing OECT models, noting that different frameworks, despite varying assumptions, can reproduce data. This question relies solely on experimental agreement for validation. We argue that models should also be evaluated on their physical principles. To assist readers, we provide COMSOL.mph files for 1D and 2D OECT models for device design and optimization.