In the given research work, experiments with optical emission spectroscopy combined with actinometry were performed in capactively coupled 13.56 MHz Ar-O2 discharge in the pressure range of 0.2 to 0.5 mbar. The Ar content in the gas mixture was varied from 5 to 30; effect of the filling gas pressure and Ar content on the variation of emission line intensities from various species, dissociation fraction and electron temperature was investigated at 150 and 250 W input powers. The electron temperature was increased with an increase in Ar content in the mixture and input power, however, a linear decrease in electron temperature was seen with an increase in filling gas pressure. It is worth mentioning here that the electron temperature increased linearly with an increase in Ar content from 5 to 10, thereafter reached its steady state. The excitation temperature was increased from 0.86 to 1.33 eV with an increase in Ar content from 0 to 30 at 0.2 mbar pressure and 250 W RF power. The presented work concludes that the electron temperature and generation of the active species strongly depend on Ar content in the mixture together with the discharge conditions. © 2015 Pleiades Publishing, Ltd.