We combine ab initio density functional theory calculations with the equivalent cores approximation1 to determine core-level binding energy shifts at phosphorus sites caused by oxidation of phosphorene2,3.We find that presence of oxygen increases the core-level binding energies of P atoms and expect binding-energy shifts of up to 6 eV in highly defective geometries.We have identified likely binding geometries of oxygen that help to interpret the observed core-level photoemission spectra in samples at different stages of oxidation and allow us to determine the fractions of specific local geometries.