In this paper, the oxidation behavior of Ti-15Mo-xO (x=0.1, 0.2, 0.3, 0.4, 0.5, mass fraction, %) alloys at 873 K and 1073 K, and the precipitation behavior of α phase at 837 K were investigated by using OM, XRD, EPMA, Vickers hardness tester and first-principles calculation. The results showed that the oxidation products were all composed of TiO2, Ti3O and Ti6O at 873 K, the oxygen contents had no significant effect on the oxidation behavior of the alloys, which was mainly controlled by the diffusion of Titanium out of the substrate. At 1073 K, the oxidation products of each alloy were TiO2, but the oxygen contents had a significant effect on the oxidation behavior of the alloys: in low oxygen-added alloy, the oxidation process was controlled by the oxidation reaction between oxygen and substrate combined with the oxygen diffusing into the substrate, while with the oxygen contents increased, it was controlled by the oxygen diffusing into the substrate, which enhanced the oxidation resistance. The α phase precipitated by ageing at 873 K for 10 h and 100 h increased with the increase of oxygen contents and ageing time, the lattice parameter of molybdenum-rich and oxygen-poor β phase decreased, and the c axis of oxygen-rich and molybdenum-poor α phase increased, while the a axis was basically unchanged, and the c/a radio increased. The hardness of the alloys increased with the increase of oxygen contents before and after ageing, however, under the condition of the same oxygen content, molybdenum and oxygen elements were redistributed in the two phases with the precipitation of α phase, which leaded to the decrease of the hardness of the alloys after ageing.