Despite extensive research into understanding the reaction mechanism for CO oxidation over transition metals supported on TiO2, the active species for oxidation remains controversial. Herein, the characteristics of the active oxygen species of blue TiO2 with a higher concentration of oxygen vacancies as a model catalyst with deposited nano-sized Pt toward CO oxidation are unraveled. Pt deposited on blue TiO2 showed 8.8 times higher catalytic activity than that on TiO2 at 200 degrees C. Formation of the oxygen vacancies induced electron generation, and the electrons were transferred to CO, weakening the binding strength. The ratio of lattice oxygen on the top surface of Pt/blue TiO2 decreased from 50.4% before the reaction to 6.1% during the reaction, as analyzed by near ambient pressure X-ray photoelectron spectroscopy. The results directly show that the facile reducibility of surface lattice oxygen of blue TiO2 leads to the high activity of CO oxidation.