In this article, we investigate the effects of oxygen partial pressure in the deposition chamber on the optical properties of zinc oxide (ZnO) thin films; in particular, we examine the variation of the refractive index with oxygen flux. ZnO thin films were deposited by radio-frequency (RF) magnetron sputtering and studied by means of X-ray diffraction (XRD) and spectroscopic ellipsometry (SE). We have found a preferential c-axis growth of ZnO films, with slightly variable deposition rates from 2.6 to 3.8 Å/s. Conversely, the refractive index exhibits, from ultraviolet (UV) to near infrared (IR), a considerable and almost linear variation when the oxygen flux value in the deposition chamber varies from 0 to 10 sccm.