Li-Mn spinel oxide cathode materials were prepared by annealing of Li-Mn spinel oxide containing an oxygen deficiency, which were synthesized by high-temperature calcination, under various oxygen pressures. The effects of the annealing conditions on the produced phase were studied. From the changes of the lattice constant and the chemical composition, it was found that the spinel oxides prepared by high-temperature calcination were oxidized by the low-temperature annealing. The change in the oxidation degree during the low-temperature annealing decreased with an increase in the Li/Mn molar ratio. The effect of the preparation conditions on the reversible Li-intercalation/deintercalation reaction was also examined from the view points of the oxygen deficiency. These experimental results implied that the cycle performance in the oxygen-deficient Li-Mn spinel oxide was improved by the low-temperature annealing, and that the cycle performance of Li-Mn spinel oxide cathode depended on the oxygen stoichiometry rather than the Li/Mn molar ratio. However, it was found that the complete recovery of the oxygen defect structure was impossible when the oxygen deficiency was large enough.