Efficient and durable catalysts towards oxygen evolution reaction (OER) are highly desirable for renewable energy technologies. Herein, we develop a new, efficient, economical and durable Ta2O5 OER catalyst by controlling oxygen environment to induce the active OER catalytic surface. Stable surface structures of Ta2O5 and their onset overpotentials towards OER are calculated via systematic DFT simulations. Oxygen site on the stable (200) surface of Ta2O5 is explicitly identified as the most active OER site, possessing a small calculated onset overpotential of 0.25V. In addition, using the simulated oxygen-rich condition, we successfully grow a (200)-surface-exposed Ta2O5 nanolayer on a carbon cloth by oxygen-controlled pulsed laser deposition. In accordance to the simulation, the designated (200) surface exhibits high OER activity with an onset overpotential of 0.29V and overpotential of 0.385V at 10mA/cm2. The oxygen-controlled surface modulation to activate catalytic activity provides a novel strategy for effective development of metal-oxide catalysts for clean energy technologies.