In this work, the influence of varying mixing ratios of O-2 on positive streamer initiation in supercritical (SC) CO2 at different applied voltages was investigated using a 3-D particle model. Zener's model was included in the 3-D particle model to calculate the field ionization rate during streamer formation and propagation. The evolution of positive streamer was classified into three main stages namely inception cloud, primary streamer channels, and successive streamer branching. The effect of O-2 with different mixing ratios (2%, 10%, and 20%) on the morphology, average electron density, electric field distribution, streamer length, and electron energy distribution function (EEDF) for positive streamer in SC-CO2 at four different voltages (12, 12.5, 13, and 13.5 kV) is then investigated. The 2-D cross sections of the initial stage of the streamer and the electric field distribution are presented in detail. At the same applied voltage, the size of inception cloud, average electron density, electric field, and streamer length with the maximum electric field decreased with increasing oxygen concentration in SC-CO2. With increasing applied voltage, the apparent size of inception cloud, primary streamer channels, successive streamer branching, average electron density, electric field, and streamer length increased. The simulation results showed that adding O-2 to SC-CO2 can significantly affect the streamer initiation and propagation.