A combination of energy-filtered electron diffraction, x-ray photoelectron spectroscopy, electron energy-loss spectroscopy, field-emission scanning electron microscopy, and x-ray diffraction are used to establish that oxygen impurities incorporated in the tungsten films prepared by magnetron sputtering play a dominant role in the formation of the stacking faulted A15 W structure. Energy-filtered electron diffraction data collected from A15 films were Fourier transformed to a reduced density function (RDF), which is compared to theoretical calculations based on several possible structural models. By using a reliability R-factor analysis the A15 W structure has been determined to be a mixed phase consisting of ordered and stacking faulted W3W structures. The effect of oxygen in stabilizing the stacking faulted A15 structure was also elucidated by in situ anneal and discussed on the basis of structural and thermodynamic stability.