Silver films with thickness between 2 and 500 nm were exposed to an excited gas mixture of argon and oxygen from a microwave ECR ion source. Oxidation was observed, even with the extraction optics grounded, with a positive or negative bias voltage on the film, and without direct sight between the film and the source. Thus, the reaction must be caused by neutral oxygen species, probably atoms. The oxygen uptake was monitored with a quartz microbalance, and yielded, after a delay of ambiguous origin, a linear increase with time until all Ag had reacted to nominally Ag2O. The rate strongly depended on the gas pressure. Simultaneous measurements of the optical transmittance, reflectance, and of the electrical resistivity suggested a layer growth mode of the oxide. Comparison of the experimental dependence of the transmittance on film thickness with computer simulations yielded a best fit with n=2.5+/-0.1 and k=0.11+/-0.02 for the oxide. Further oxygen absorption was slower, and approached an equilibrium of the production and decay of AgO.