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The characteristics of RF planar magnetron discharges of O2/Ar mixture are clarified using the particle-in-cell/Monte Carlo method. The simulation is carried out for axisymmetrical magnetic fields. The spatial and temporal behavior of magnetron discharge is examined in detail. The O- ions are trapped in the plasma bulk by the time-averaged potential. The O- density is governed by a balance of generation and loss of O-. The positive ions Ar+, O2+, and O+ are distributed in such a way that the charge neutrality is kept in the plasma bulk. Since the peak point of O- density does not coincide with that of electron density, the spatial distributions of Ar+, O2+, and O+ have two peaks. The number densities of Ar+ and O- ions drastically increase when oxygen is added to the discharge gas Ar, and then saturated. In the case when the mole fraction of oxygen increases, the self-bias voltage on the target decreases. This phenomenon is investigated in detail and a clear physical explanation is presented.