The dynamics of the expansion plasma produced by excimer laser ablation of a silicon target into oxygen and mixed O2/Ar atmosphere were studied by means of time-resolved imaging of the expanding plume. Experiments were performed in pure oxygen, ranging between 0.13 and 13.33 Pa, and at different O2/Ar ratios at a fixed total pressure of 13.33 Pa. The occurrence of a shock wave (SW) generated by the supersonic expansion of the plasma was observed at high pressure values. The presence of the SW had a strong influence on the structure of SiOx thin films. In fact, silicon dioxide thin films were always obtained in presence of the SW, irrespective of the oxygen content in the gaseous mixture. On the contrary, suboxide thin films were obtained when the expansion occurred at lower pressure values (no SW presence). The temperature rise following the developing of the SW, is supposed to enhance the oxygen molecules dissociation by increasing the efficiency of the silicon oxidation reaction.