The chemical stability of Li metal in a rechargeable Li–O2 cell was examined by investigating the physicochemical changes that occurred during storage of Li in an electrolyte comprising dimethyl sulfoxide (DMSO) with dissolved O2. During prolonged storage of Li in the oxygenated electrolyte, the Li surface became moss-like and its interfacial resistance increased. Analyses of reaction products using XPS and FT-IR revealed that the bis(trifluoromethyl sulfonyl) imide (TFSI) anions and DMSO solvent could have decomposed significantly through a further reaction path induced by O2. Furthermore, the formation of an unstable solid-electrolyte interphase by O2 causes degradation of the Li metal and deterioration of the electrolyte. This investigation shows that the Li anode should be protected from O2 by modification of Li and the separator to ensure long-term stability of the Li–O2 cell.