<p>Extracorporeal membrane oxygenation (ECMO) is a life-support modality that supports cardiopulmonary function in critically ill patients. Veno-arterial (VA)-ECMO, which reinfuses blood through the femoral artery, can cause upper body hypoxemia due to uneven oxygen distribution within the aorta. In this study, computational haemodynamic simulations were performed using a patient-specific aorta geometry to simulate blood flow and quantify oxygen saturation (SO₂) levels in the arch branches under varying ECMO support levels. A single-phase flow model coupled with oxygen transport was employed and compared to the multiphase flow model used in previous studies. Simulation results were analysed to evaluate the locations of watershed zones formed by the interplay of native cardiac and ECMO flows and the corresponding oxygen levels. Our findings demonstrate that the single-phase model predicted IA SO₂ ranging from 71.1% to 94.2%, whereas the multiphase model predicted 70.0–86.7%, indicating an underestimation of oxygen saturation in the aortic arch branches. Additionally, lower ECMO support levels shift the watershed region distally, reducing oxygen delivery to the arch branches. This study highlights the potential of computational haemodynamic simulations for assessing oxygen transport and haemodynamic behaviour in patients undergoing VA-ECMO. The approach provides insights to support clinical decision-making and improve personalised treatment outcomes.</p> Graphical Abstract <p></p>