[en] Interband optical transitions in highly mismatched ZnSe_1−xO_x alloys with up to 1.35% O content have been studied with photoreflectance. Incorporation of oxygen results in a pronounced reduction of the temperature dependence of the energy gap of the alloy. A detailed analysis of the experimental data shows that the reduction in the temperature dependence of the band gap with increasing O content can be explained by the band anticrossing interaction between the temperature dependent conduction band of ZnSe host matrix and the temperature independent on the absolute scale energy highly localized O states. It has been shown that the assumption of constant energy separation between the oxygen level and the valence band cannot be applied in order to achieve good quantitative agreement of the theoretical and experiment data. (paper)