γ－TiAl intermetallics have attracted much interest in the last decade as prospective structural materials for aerospace applications, since they maintain a large number of outstanding properties, such as high melting point, low density and high－temperature strength. However, the oxidation resistance of TiAl alloys is inadequate at high temperature, hindering their practical applications. In order to improve the oxidation resistance, an understanding of the growth mechanisms of these surface oxides is of great importance. The adsorption and diffusion of oxygen atom at the surface are the important processes in the oxidation. In this work, the effects of point defects on the adsorption and diffusion of oxygen at γ－TiAl (100) surface are studied by the means of first－principles calculations. The calculated results of the formation energies of the point defects show that in the doped γ－TiAl (100), the alloying Si atom prefers occupying the first surface layer Al site, while W prefers to occupy the second sublayer Ti site at theγ－TiAl (100) surface, and both of them increase the absorption energies of oxygen atom, which is the near neighbour of them. It indicates that Si prefers to segregate at the first surface layer and W prefers to segregate at the second sublayer, and both of them can suppress the adsorption of oxygen atoms at the γ－TiAl (100) surface. In the defect of vacancy system, the Ti vacancy is easier to be formed than Al vacancy on the first surface layer. Furthermore, the diffusion of oxygen atom at the γ－TiAl (100) surface are studied by the method of Climbing－image Nudge Elastic Band. The calculated diffusion barriers of oxygen atom from the position on the surface to the second sublayer in these systems of perfect, Ti vacancy, Si and W doped are 1.98, 1.34, 2.53 and 2.69 eV, respectively. It indicates that the diffusion of oxygen atom is made easier by the Ti vacancy, while it is more difficult by the dope of Si and W.