The saturated oxide formed on Ni(001) under UHV conditions has been studied by x-ray photoelectron diffraction (XPD) and low-energy electron diffraction. At ambient temperature and with a 1200-L exposure to oxygen, the saturation oxygen coverage is found to be 4.30.4 monolayers (ML) as measured relative to the Ni(001) surface-atom density. [1 langmuir (L)==10-6 Torr sec.] The oxide is furthermore found to grow primarily as NiO(001) in a highly strained superlattice for which the horizontal lattice constant is expanded by (1/6 compared to the underlying Ni(001). The XPD results are found to be very sensitive to the degree of short-range order in the oxide before and after light annealing, particularly when obtained with high angular resolutions of approximately 1.0°. Single-scattering calculations with spherical-wave scattering are found to give an excellent description of the XPD from the annealed and more-ordered overlayer. These results also suggest the general utility of high-resolution XPD for studying the degree of short-range positional order present in epitaxial overlayers. © 1989 The American Physical Society.