This study explores the impact of oxygen vacancy defects on elevated-metal metal-oxide (EMMO) IGZO TFTs under positive bias stress (PBS) using TCAD and DFT simulation. Findings reveal that oxygen vacancies accumulating at the channel/passivation layer interface and within the channel under PBS lead to negative threshold voltage shifts and reduced mobility. Additionally, higher tail state densities contribute to a positive Vth shift. These results provide important insights into the defect-related reliability of EMMO IGZO TFTs, guiding the design of more reliable devices.