In this study, CdTe (up to 2.0 mu m thick) and oxygenated CdS (CdS:O, up to 100 nm thick) films were deposited by magnetron sputtering and optimum conditions of film growth were investigated for CdS:O/CdTe solar cells. Favourable TeO2 has been confirmed in XRD after the CdCI2 heat treatment of the CdTe films. Moreover, improved structural, optical and electrical properties are observed in the CdCI2 heat treated films. A detailed quantitative study has also been executed using XPS that finds sulfide, sulfate and an intermediate oxide as a function of oxygen content. In many cases, CdS contribution remains predominant, however, the CdSOX contribution increases with the increase of oxygen's partial pressure and decrease of growth rate. The complete solar cell device was fabricated of various CdTe thin films with different growth rates in sputtering. A highly resistive transparent (HRT) buffer layer ZnO:Sn was placed in between the FTO and CdS:O to avoid the forward leakage problem and screen printed C:Cu/Ag is used as the back contact for low cost fabrication. The J-V characteristics and external quantum efficiency (EQE) were measured for the solar cells under the illumination of AM 1.5G and the highest efficiency of 10.3% was achieved for the optimized CdTe growth rate of 5.4 angstrom/s, while CdS:O growth rate was 0.25 angstrom/s.