Oxygen barrier coatings were made by plasma polymer deposition on a polypropylene substrate. Plasma polymer deposition is realized in a low-frequency reactor with hexamethyldisiloxane/oxygen (HMDSO/O-2) precursors. The structure and composition of the deposited coating are characterized by density measurements and FTIR spectroscopy. Permeability of the polypropylene, coated with plasma films made from various gas compositions, is interpreted in terms of coating structure and composition. Incorporation of molecular oxygen in the gas phase results in inorganic and dense films. It is shown that the density of the film is the main factor controlling the oxygen barrier effect.