Mixtures of hexamethyldisiloxane [HMDSiO, (CH3)(3)SiOSi(CH3)(3)] and oxygen are plasma polymerized at different oxygen pressures (P-O2 = 1.3-11.4 Pa) and a fixed monomer pressure (P-m = 2.6 Pa). The discharge power is kept at 100 W throughout the work. Nanometer-size holes in the deposited films are characterized by variable-energy positron annihilation lifetime spectroscopy (PALS). Additional information on the film composition and structure is obtained by X-ray photoelectron spectroscopy and IR absorption spectroscopy. The ortho-positronium lifetime tau (3) and intensity I-3 increase with the P-O2 up to 6.2 Pa and then decrease with the P-O2. PALS measurements after annealing at 400 degreesC show that films prepared at high oxygen pressure have a less stable structure than a film deposited at a lower oxygen pressure. These results are discussed in comparison with plasma deposition of pure HMDSiO, as are the possible effects of oxygen radicals on the film structure.