The integration of 4 nm thick amorphous indium tungsten oxide (a-IWO) and a hafnium oxide (HfO2 ) high-κ gate dielectric has been demonstrated previously as one of promising amorphous oxide semiconductor (AOS) thin-film transistors (TFTs). In this study, the more positive threshold voltage shift (∆VTH ) and reduced ION were observed when increasing the oxygen ratio during a-IWO deposition. Through simple material measurements and Technology Computer Aided Design (TCAD) analysis, the distinct correlation between different chemical species and the corresponding bulk and interface density of states (DOS) parameters were systematically deduced, validating the proposed physical mechanisms with a quantum model for a-IWO nanosheet TFT. The effects of oxygen flow on oxygen interstitial (Oi ) defects were numerically proved for modulating bulk dopant concentration Nd and interface density of Gaussian acceptor trap NGA at the front channel, significantly dominating the transfer characteristics of a-IWO TFT. Furthermore, based on the studies of density functional theory (DFT) for the correlation between formation energy Ef of Oi defect and Fermi level (EF ) position, we propose a numerical methodology for monitoring the possible concentration distribution of Oi as a function of a bias condition for AOS TFTs.