The efficient reversible formation of discharge products for Li-O2 batteries is still challenging. Meanwhile, the question of the nature of the discharge products and their decomposition mechanism are still remain. Implanting oxygen vacancies on the metal oxides can create negative-charge surface to provide strong adsorption of active oxygen and at the same time, the exposed metal sites can serve as an efficient substrate for decomposed reaction of discharge products. In this work, we apply the graphene-like MoO3 ultrathin nanosheets as a matrix. By controlling the reduction time, the MoO3 nanosheets with different concentration of oxygen vacancy are obtained. Experimental results reveal that the MoO3 nanosheets with the high-concentration oxygen vacancies can significantly decrease the overpotential and get enhanced electrochemical response, namely, a low overpotential of ~0.5V can be delivered with ultra-stable cycles (over 60 cycles). Moreover, the Li-O2 batteries demonstrate an interesting four-step discharge and charge process. X-ray photoelectron spectroscopy, X-ray diffraction, transmission electron microscopy and electron energy-loss spectroscopy analyses are carried out for the four typical states of the cathode to reveal the reaction mechanism for the unique electrochemical behavior.