The influence of oxygen on solidification microstructure of high Nb containing TiAl alloys with peritectic liquid composition was experimentally investigated. Alloys with oxygen content from 0.1 to 1.0 at.% were prepared by non-consumable vacuum arc remelting under protective atmosphere. Results indicate that increasing oxygen content in Ti–56Al–10Nb–xO alloys remarkably increases the amount of α2 phase, and shifts the peritectic reaction α + L → γ to a higher Al content. In Ti–54Al–8Nb–xO alloys, the mixed dendritic microstructure forms. The fine dendrites show peritectic halos and coarse dendrites consist of lamellae. The mixed dendritic microstructure has been interpreted by the stabilizing effect of oxygen on α phase and lamellar structure in high Nb containing TiAl alloys.Highlights► Effect of oxygen on the microstructure in high Nb-TiAl alloys were examined. ► Increasing oxygen moves the peritectic reaction to a higher Al content. ► Increasing oxygen moves the eutectoid reaction to a higher Al content. ► Increasing oxygen leads to a mixed dendritic microstructure.