A global plasma model is used to investigate the chemistry and energy deposition in 13.56 MHz radio-frequency capacitively coupled oxygen-argon discharges under conditions usually used for the deposition of tin oxide films. These models are based on the solution of a stationary electron Boltzmann equation coupled to species balance equations including the vibrational kinetics equations of O-2. The results obtained showed that vibrational non equilibrium of O-2-molecule is not significant. The dissociation degree of O-2 was found to be around a few percents and the discharge was often moderately electronegative even for small O-2 contents in the feed gas. The ionization and energy dissipation mechanisms are mainly governed by the collisional processes involving O-2 for an oxygen feed gas composition greater than 20%. Results also showed that the predicted densities of the charged species and the electronegative character of the discharge are strongly linked to the assumption made on the homogeneity of the power deposition. On the contrary, the predicted density of O-atom is not sensitive to this assumption.