The phase equilibria and thermodynamic properties of the binary Ba-O system were analyzed. The Gibbs energy functions of individual phases were modeled, and the model parameters were obtained by means of the CALculation of PHAse Diagram (CALPHAD) technique, based on available experimental data in the literature. The liquid phase was described with a two-sublattice ionic model. For the non-stoichiometric BaO and BaO2 phases, a two-sublattice ionic model was applied, and two sets of Gibbs energy functions were obtained by treating them either as separate phases or as a common solution phase with a miscibility gap. The ionic sublattice models used in the present work provide an approach to better describe the physical behavior of the Ba-O binary system such as defect chemistry and doping effects, which will be studied in multicomponent systems.