Unlike graphene, the existence of direct bandgaps in transition metal dichalcogenides such as MoS2 offers an attractive possibility of using single layer MoS2 in transistors, sensors, optoelectronic devices, and flexible systems. Because of the absence of dangling bonds in MoS2, surface treatment such as ultraviolet-ozone (UV-O3) treatment is necessary before the deposition of high-k dielectrics on MoS2 to fabricate various devices. However, little interest has been given to the effect of surface oxidation of single layer MoS2 by UV-O3 treatment. In this presentation, we systematically investigate the effect of UV-O3 treatment on the monolayer MoS2 thin films obtained by chemical vapor deposition (CVD). We observe photoluminescence quenching in monolayer MoS2 thin films with increasing UV-O3 treatment time. We also observe the reduction and broading of MoS2 Raman modes with increasing UV-O3 treatment time. X-ray photoelectron spectroscopy indicates the nature of oxygen bonding changes with increasing UV-O3 treatment time. These results demonstrate the significant impact of surface oxidation by UV-O3 treatment on monolayer MoS2 thin films suggesting the importance of using optimized surface treatment conditions for device applications.