<p>O significantly affects the thermal/stress-induced products in β-type titanium alloys. However, the mechanism of O effects on the displacive behaviors involved in the thermal/stress-induced phase transformation and twinning&#xa0;process is still not fully elucidated. The effects of interstitial O on specific orientation moduli and thermal/stress-induced products in bcc Ti–Mo alloys were investigated through first-principles calculations combined with microstructural observations. The addition of O decreased the formation energy of the supercell and formed the Mo–O and Ti–O bonds with the increase in the bonding electron number, thereby enhancing the β phase stability. The specific orientation moduli such as Young’s modulus (<i>E</i>_<i>100</i>), tetragonal shear modulus (<i>C</i>′), and shear moduli (<i>G</i>_<i>111</i> and <i>G</i>_<i>113</i>) were increased, which was attributed to the formation of Mo–O and Ti–O bonds along &lt;100&gt;_β direction and the strengthened Ti–Ti bonds along &lt;110&gt;_β, &lt;111&gt;_β and &lt;113&gt;_β directions, respectively. The addition of O suppressed the thermal-induced ω phase, corresponding to its structural change from hexagonal to tripartite, and the predominant stress-induced product of α" martensite was inhibited due to the increase in moduli of <i>G</i>_<i>111</i>, <i>C</i>′, and <i>E</i>_<i>100</i>. This study deepens the understanding of the role of O in phase transformation and twinning mechanisms.</p>