The ordering in the Cu-O basal plane of YBa2Cu3Oz is investigated as a function of oxygen content and temperature. A phase diagram is calculated by application of the cluster variation method to an asymmetric two-dimensional Ising model. The interaction parameters selected guarantee the stability of the experimentally observed double-cell structure around z=6.5. The transition temperatures at various oxygen partial pressures are in good agreement with experiment. The pair probabilities along the Cu-O chains show a plateau with varying oxygen concentration, not unlike the dependence of Tc on oxygen content.