Ferroelectric polarization plays important roles in catalytic reactions, but the mechanism is still under debate. In this work, 2D ferroelectric MIrSnS is systematically investigated for revealing the effects of ferroelectric polarization on the oxygen evolution reaction (OER) based on density-functional-theory (DFT) calculations. We find that: (1) the adsorption configurations of intermediates, protons, and water strongly depend on the polarization, which in turn affects their bonding patterns and adsorption energies; (2) the paraelectric (PE) state generally shows the highest OER activity (overpotential = 0.54 V) due to its most favorable free energy change; and (3) the capacitances are varied under different polarization states, which affects the reaction kinetics. Our findings illustrate the full picture of the OER process under different polarizations, which may provide insightful understanding on the ferroelectric-enhanced catalytic activity and guide the design of novel ferroelectric catalysts.