In spite of several desirable properties such as high stability against superoxide anion and low vapor pressure, N-methyl-2-pyrrolidone (NMP) electrolyte is reported not suitable for use in lithium-oxygen (Li-O2) batteries because of severe degradation upon cycling and low oxygen efficiency. In this work, we find that NMP electrolyte is reactive with O2 gas in the presence of lithium metal and such O2-consuming reaction (i.e., autoxidation) is a possible cause for the poor performance in Li-O2 batteries with NMP electrolyte. The autoxidation of NMP is verified by direct measurement of the depletion of O2 gas in the hermetically sealed symmetric Li/Li cells via in-situ gas pressure analysis. In-situ differential electrochemical mass spectroscopy (DEMS) experiment reveals that the autoxidation resulted in significant O2 consumption upon discharge, very low O2 efficiency upon charge, and eventually fast capacity fading. Lithium nitrate (LiNO3), which provides a protective layer on the surface of lithium metal, is employed to suppress the autoxidation, leading to significantly enhanced oxygen efficiency and cycle life.