The effect of plasma power, reactor outgassing rate and feed gas purity on the oxygen content of amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) layers, prepared by the VHF (70 MHz) glow discharge technique has been investigated. The oxygen concentration in the films was measured by Secondary Ion Mass Spectrometry. The gas purity could be optionally enhanced by the application of a getter-based gas purifier mounted in the gas pipe just before the reactor. The purifier reduces the oxygen contaminants in the feed gas to levels in the ppb range. It was found that oxygen incorporation was enhanced at lower deposition rates. The deposition rate dependence of the oxygen incorporation is in excellent agreement with the proposed incorporation mechanism. Furthermore, the reactor outgassing rate and the purity of the feed gas itself were identified as the main contamination sources for the oxygen contaminants in the deposited a-Si:H films as well as in μc-Si:H films : e.g. at outgassing rates in the range of 10−6 mbar 1 s−1, at least around half of the oxygen detected in both a-Si:H and in the μc-Si:H layers originates from the feed gas if the latter is unpurified. By combining reduced outgassing rates and the use of purified feed gas, a-Si:H and μc-Si:H films with an oxygen concentration of 5×1017cm−3 and 2×1018cm−3, respectively, were deposited. These values both represent the lowest concentrations of oxygen contaminants reported to date for these kinds of materials.