An oxygen-doped GeSbTe interface layer improves the overwriting characteristics of the phase-change optical disk in the blue wavelength. The thermal and optical properties of oxygen-doped GeSbTe film and its crystal structure were investigated. Crystallization temperature and activation energy of the amorphous Ge–Sb–Te–O films are increased with the oxygen concentration while the melting point is decreased. The refractive index of the crystalline state monotonically increases with the oxygen concentration of the film, while its extinction coefficient monotonically decreases. In terms of the crystalline structure, fcc characteristic peaks disappear gradually with oxygen concentration, and above 35 at.% of oxygen, hexagonal peaks appear.