Thin films of amorphous FeCuSiNbB alloy have been deposited by RF sputtering with various deposition rates. The bulk oxygen content has been characterized using EDS and XPS. Its dependence on deposition rate shows that water vapour in the sputtering chamber is at the origin of the contamination. It allows also estimating the adsorption coefficient of the oxygen on the sample to be around 15 % at 350 K. The magnetic hardness and the resistivity increase with the contamination in oxygen. In devitrified films, this increase is also related to an enrichment of the residual amorphous matrix in oxygen.