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We investigate the chemically tunable properties of understoichiometric graphene oxide as a function of the growth temperature and the amount of hydroxyl and epoxy groups adsorbed on graphene. The structural disorder is modeled within a statistical approach based on a cluster expansion. A set of 308 cluster symmetry classes is investigated with ab initio calculations based on density functional theory. A complete scenario of energetics and phase stability is developed, yielding insight into the structure and electronic properties of graphene oxide. Our results show that the tendency to agglomeration of oxygen-containing groups is independent of their relative proportion and indicate the favorable formation of a unique oxygen-rich phase with both groups. Structural and electronic properties are predicted for the whole range of chemical compositions. The optical properties of oxygen-rich phases are also discussed in detail for different growth conditions.