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The authors developed a tool using a multiscale approach to simulate the silicon etching using Bosch process. Their study is focused on the analysis of the effect of the oxygen addition to C4F8 plasma during the deposition pulse. This is the complementary study that the authors have recently published which was dedicated to the Bosch process under pure SF6 plasma used in etching pulse and pure C4F8 plasma used in polymer deposition pulse. Parametric study about the effect of the oxygen percentage on the reactive species flux evolution and their impact on the deposition kinetic during the deposition pulse has been performed. The simulation results reveal that for a low %O2 in a C4F8/O2 plasma mixture, the atomic fluorine density increases because of the volume reactions, especially recombinations between CFx and O which favor the production of fluorine. This leads to the decrease of CFx to F flux ratio. Ion energy distribution functions (IEDF) plotting reveals the impact of both %O2 and mass of the positive ions on the IEDF shape. Finally, both the experimental and simulation results show that in their pressure range, the addition of O2 to C4F8 plasma has a weak impact on the silicon etch profile and the etching rate, except for a high %O2 which the etch anisotropy begins to be degraded.