Abstract In this work, Ti–23Nb–0.7Ta–2Zr (TNTZ) and gum metal Ti–23Nb–0.7Ta–2Zr–1.2O (TNTZ-O) alloys were synthesized by cold crucible levitation melting with the objective of investigating the influence of oxygen on the deformation mechanisms. By tensile tests, electron backscatter diffraction, atomic force microscopy and transmission electron microscopy analyses, we showed that the deformation in the TNTZ-O alloy is only accommodated by dislocation slip. Thus, the addition of oxygen suppresses the formation of α″ martensite and prevents the twinning deformation mechanism, which were both observed in the TNTZ alloy. In addition, in situ tensile tests in a transmission electron microscope showed that conventional a/2〈1 1 1〉 dislocation slip occurs widely in the TNTZ-O alloy. Screw dislocations have a lower mobility than non-screw dislocations. Cross-slip is shown to be easy and multiplication of dislocations by a double cross-slip mechanism occurs extensively, leading to the formation of large slip bands.