Thin organosilicon and silicon oxide films were deposited in r.f. capacitively coupled discharges from a mixture of hexamethyldisiloxane (HMDSO) and oxygen. The concentration of HMDSO was in the range 5-17%. Even for such relatively high dilution of HMDSO the organic-inorganic crossover of the film character was observed due to changes of the HMDSO concentration, but other factors, such as pressure and d.c. self-bias should also be taken into account. When annealed the films changed their composition, chemical structure and mechanical properties. We observed desorption of water, methane and CO or CO2 from the SiO2-like films. The hardness of the SiO2-like film, containing 5% carbon and 25% hydrogen, increased with the increase of annealing temperature from 5.9 (as deposited) to 11.3 GPa (500 oC). Simultaneously, its fracture toughness was significantly improved. These effects were explained by dehydration and cross-linking of the film. However, the mechanical properties of highly cross-linked SiO1:9C1:6H0:6 plasma polymer were superior over the SiO2-like film containing impurities and such film can be used as a protective coating with a hardness above 9.6 GPa up to the temperature of 400 oC.