NiO films of different thickness (0.8-12 ML) were grown in situ on Ag(100) and studied by low-energy electron diffraction, X-ray photoemission, X-ray photoelectron diffraction and X-ray fluorescence spectroscopy. The photoemission and X-ray fluorescence spectra recorded from epitaxial NiO(100) films provide evidence for the sensitivity of the inner-shell electronic transitions to the geometry of the first and the second coordination spheres around the absorbing Ni2+ ion. The relative intensities of individual structures in the Ni 2p photoemission spectra change significantly with increasing thickness of the film. They are supposed to be influenced by competition between transitions of screening electrons coming either from the surrounding ligands or from ligands around a neighboring transition metal ion. These findings are consistent with the results of theoretical studies of non-local screening effects explaining the shape of the main line in the Ni 2p spectra. Information about local partial densities of states was obtained from a comparative analysis of Ni L and O K X-ray fluorescence and valence-band photoemission spectra.