Titanium alloys can suffer from halide-associated stress corrosion cracking at elevated temperatures e.g. in jet engines, where chlorides and Ti-oxide promote the cracking of water vapour in the gas stream, depositing embrittling species at the crack tip. Here we report, using isotopically-labelled experiments, that crack tips in an industrial Ti-6Al-2Sn-4Zr-6Mo alloy are strongly enriched (>5 at.%) in oxygen from the water vapour, far greater than the amounts (0.25 at.%) required to embrittle the material. Surprisingly, relatively little hydrogen is measured, despite careful preparation. Therefore, we suggest that a synergistic effect of O and H leads to cracking, with O playing a vital role, since O is well-known to cause embrittlement of the alloy. In contrast it appears that in alpha-beta Ti alloys, it may be that H may drain away into the bulk owing to its high solubility in beta-Ti, rather than being retained in the stress field of the crack tip. Therefore, whilst hydrides may form on the fracture surface, hydrogen ingress might not result in embrittlement of the underlying matrix. This possibility challenges decades of understanding of stress-corrosion cracking as being related only to the hydrogen enhanced localised plasticity (HELP) mechanism, which explains why H-doped Ti alloys are embrittled. This would change the perspective on stress corrosion embrittlement away from a focus on hydrogen towards the ingress of O originating from the water vapour, insights critical for designing corrosion resistant materials.