Oxygen enhances the strength of titanium alloys in general; however, excess oxygen can make titanium alloys brittle. On the other hand, oxygen enhances the precipitation of the a phase and suppresses the formation of the ω phase. Thus, using the optimal amount of oxygen is important to improve the mechanical properties of titanium alloys. The role of oxygen in titanium alloys is still not well understood. The effect of oxygen on the mechanical behavior of a β-type titanium alloy, Ti-29Nb-13Ta-4.6Zr (referred to as TNTZ), which is used for biomedical applications, was investigated in this study. Oxygen was found to stabilize the ω phase in TNTZ. This behavior of oxygen is unusual considering the known behavior of oxygen in titanium alloys: oxygen is known to suppress the formation of the ω phase in titanium alloys. A small amount of oxygen increases the tensile strength but decreases the ductility of TNTZ. On the other hand, a large amount of oxygen, of around 0.7 mass%, increases both the tensile strength and the ductility of TNTZ. This phenomenon is unexpected.