β-type alloys have a wide application prospect in aerospace, biomedical and marine engineering and other fields, owing to their high specific strength, good corrosion resistance and low elastic modulus. Their yield strength and uniform elongation are affected by the second phase precipitation, plastic deformation mode and interstitial element, especially the oxygen element. In this work, the effect of tensile pre-deformation induced {332}<113> twins combined with isothermal ω-phase after subsequent ageing on the mechanical properties of β-type Ti-15Mo alloy with different oxygen contents from 0.1% to 0.5% (mass fraction) was examined by OM, XRD, TEM and DSC, Vickers hardness tester and tensile testing machine. The results indicated that with increasing the oxygen content, the formation of mechanical twins and isothermal ω-phase in the alloy was suppressed, and the effect of pre-deformation induced twins on the precipitation of isothermal ω-phase was negligible. After pre-deformation combined with subsequent ageing, the alloy with low oxygen content had the relatively high yield strength and large uniform elongation, but it with high oxygen content exhibited the brittle fracture. A good combination of strength with ductility in the alloy with low oxygen content was contributed to the twinning and dislocation slip coupled deformation. The high yield strength was mainly dominated by the dislocation slip, and the large uniform elongation was due to the static and dynamic grain refinement effects, which were caused by the pre-deformation induced twins and subsequent twinning deformation, respectively. Through utilizing the alloying element of oxygen effectively, and changing the plastic deformation mode and phase precipitation behavior based on the reasonable process of pre-deformation and heat treatment, the combination of strength and ductility can be controlled in a large range for the β-type titanium alloys.