Unlike graphene, the existence of direct bandgaps in transition metal dichalcogenides such as MoS2 offers an attractive possibility of using single layer MoS2 in optoelectronic devices. 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 optoelectronic devices such as phototransistors. However, little interest has been given to the effect of UV-O3 treatment on the optoelectronic properties of single layer MoS2. In this presentation, we systematically investigate the effect of UV-O3 treatment on the photoluminescence of mechanically exfoliated single layer MoS2 flakes. We observe photoluminescence quenching in single layer MoS2, accompanied by reduction and broading of MoS2 Raman modes with increasing UV-O3 treatment time. X-ray photoelectron spectroscopy confirms the formation of oxygen bonding. We demonstrate that the formation of oxygen bonding upon exposure to UV-O3 treatment leads to a direct-to-indirect bandgap transition in single-layer MoS2. These results also demonstrate the significant impact of UV-O3 treatment on the optoelectronic properties of single layer MoS2 suggesting the importance of using optimized process conditions.