Large eddy simulations of pre-designed micro-hydrokinetic turbine were conducted to investigate the oxygen transfer from air to water. Simulations were performed in extreme conditions having a tip-speed ratio of 3.8 that is higher than the tip-speed ratio at turbine's design point. Air was injected from the turbine hub downstream in axial direction. Both single phase and multiphase simulations were performed to reveal the influence of air admission on the flow structures and the turbine performance. The mixture multiphase model was employed in multiphase simulation. The results indicated that turbine power generation was reduced roughly 10.5% by air admission, however the torque applied on turbine surface in axial direction did not vary significantly by aeration. The aeration assisted in the suppression of vortices within the flow field. The deviation of the power coefficient and the thrust coefficient was reduced roughly 32% through the inclusion of aeration process.