Ga-assisted growth of GaAs nanowires on silicon provides a path for integrating high-purity III-Vs on silicon. The nature of the oxide on the silicon surface has been shown to impact the overall possibility of nanowire growth and their orientation with the substrate. In this work, we show that not only the exact thickness, but also the nature of the native oxide determines the feasibility of nanowire growth. During the course of formation of the native oxide, the surface energy varies and results in a different contact angle of Ga droplets. We find that, only for a contact angle around 90 degrees (i.e., oxide thickness similar to 0.9 nm), nanowires grow perpendicularly to the silicon substrate. This native oxide engineering is the first step toward controlling the self-assembly process, determining mainly the nanowire density and orientation.