Plasma enhanced chemical vapour deposition (PECVD) of thin oxide films was investigated, changing electrical potential conditions at the substrate electrode (dc, rf coupled or positive biased). Rf discharge at the frequency of 13.56 MHz was generated in a planar reactor with two internal electrodes. Silicon substrates were placed on the powered electrode. The octamethylcyclotetrasiloxane (OMTS) was chosen as a source of O-Si-O groups in order to test new possibilities in the silicon oxide depositions. The reflectance in the visible, the transmittance in the infrared region, X-ray photoelectron spectra (XPS) and Rutherford backscattering method (RBS) analyses were applied to describe the deposition rate, the optical properties, the composition and the structure of the deposited films. The comparison among these four methods concerning the film composition and structure is discussed. The pronounced changes in the deposition rate with rf power or de bias typical of every electrical potential condition were observed. However, other him characteristics seemed to be very similar. The film optical parameters and the atomic composition were close to those of amorphous silicon dioxide.