Ferroelectrics hold great promise in catalysis because of their ability to modulate intermediate adsorption strength and facilitate electron transfer on polarized surfaces. However, the effect of ferroelectric polarization on catalytic performance remains unclear. Here, an epitaxial BaTiO film was fabricated by pulsed laser deposition, which was utilized as a proof-of-concept material to reveal the polarization-dependent oxygen evolution reaction (OER). We show that negative ferroelectric polarization significantly improves the OER performance, due to enhanced adsorption of OH and accelerated charge transfer by the redistribution of oxygen vacancies, and the additive effect of the built-in electric field. Interestingly, we find a periodic oscillation in the chronoamperometry test, which is probably attributed to the adsorption of reactants, gradual accumulation and sudden release of products, and the dynamical evolution of ferroelectric polarization. This work not only provides valuable insights into the origin of the controllable OER activity of ferroelectrics, but also sheds light on the relaxation of ferroelectric polarization in catalytic processes.