A clear understanding of the surface chemical reactivity of tungsten (W) films is indispensable for photocatalytic, sensing and memory applications, especially in the presence of WO x (0 ≤ x ≤ 3) for low thicknesses. Here, surface reactivity of the film through diffusion of oxygen and its ability to make bonds with W is identified by X-ray photoelectron spectroscopy (XPS). Further inspection of XPS valence band spectra confirms the possible hybridization of W 5d and O 2p electrons in the presence of defect states near Fermi level. Exploration of surface morphology by scanning electron microscopy (SEM) reveals agglomeration of grains with increasing film thickness. Detailed microstructural and grazing-incidence X-ray diffraction (GIXRD) studies suggest the formation of β W nanocrystallites in amorphous matrix, and establish a knowledge of thickness dependent phase transformation of W beside surface oxidation. The nonlinear surface current–voltage characteristics at low thickness further indicates dimensional phase change owing to the involvement of point defects. We also report a detailed study of interstitial and vacancy mediated diffusion probability of oxygen in W films, where the estimated diffusion constant is found to be relatively higher than that of other body-centered cubic transition metals. © 2019, Springer Science+Business Media, LLC, part of Springer Nature.