In this paper we report a detailed investigation of ZnO thin film properties deposited on Si<100> substrate at 400°C using RF sputtering. To reduce oxygen induced vacancies and interstitial defects in samples, variable oxygen flow rate during deposition followed by post growth annealing in oxygen ambient were carried out. Four samples were deposited under constant temperature condition but with variable oxygen partial pressure of 0%, 20%, 50% and 80% in Argon and Oxygen mixture, namely sample S1, S2 , S3 and S4 respectively. Deposited films were further annealed at 700, 800, 900 and 1000°C in oxygen ambient for 10s. Photoluminescence (PL) measurements carried at low temperature (18K) demonstrated near band edge emission peak of ZnO at 3.37eV. Increment in PL intensity was observed with increasing annealing temperature and a particular sample S4 annealed at 900 measured narrowest full width half maxima (FWHM) of ~0.1272eV. Defects peaks observed at lower energies were suppressed with increasing oxygen flow and post growth annealing, indicating improvement in film quality. From HRXRD measurement it was observed S4 sample annealed at 900°C has the highest peak intensity and narrowest FWHM compared to other samples, demonstrating the best crystalline property of annealed film at 900°C. Highest XRD peak intensity measured at 34.53° corresponds to (002) crystal orientation reveals that the films were highly caxis oriented. AFM results show increase in grain size with increasing oxygen flow and annealing temperature which ensures improvement in morphological properties of the film.