Silicon oxide based (SiO_xC_yH_z, noted SiO_x) coatings are often used in surface engineering for microelectronics, corrosion resistance, barrier to gas permeation through polymeric materials, etc. . . SiO_x coatings can be efficiently deposited by non-equilibrium atmospheric pressure plasma processes, such as DBD or plasma jets. In the present study, the design of experiment (DoE) methodology was used to investigate the influence of process parameters on the characteristics of organosilicon coatings deposited by a non-equilibrium atmospheric pressure plasma jet (APPJ) from hexamethyldisiloxane (HMDSO) and air mixtures. The results obtained were used to create an empirical model to predict the chemical composition of coatings. Among 11 process parameters, the 3 parameters which exhibited the strongest effect on the coating composition were the torch speed, the substrate to nozzle distance and the number of scans. Auger spectroscopy revealed that the carbon content of the thin films was as low as 6±1.7% and AFM analysis showed that smooth coatings (R_a ˜ 2 nm) were obtained even at high dynamic growth rates (˜1 000nm.m. min^-1). A tentative macroscopic scaling law was also formulated to correlate our results with the available literature data.