Natural convection and oxygen transfer characteristics in square cavity subjected to the magnetic field are studied numerically. Oxygen transfer in liquid metals has attracted much attention because it can decrease the corrosion rate of steel in contact with liquid metals. In advanced reactors, liquid lead has been utilized as an effective coolant. As indicated by many research reports that corrosion could be decreased by controlling the proper level of oxygen in the liquid lead. In this method, oxygen needs to mix in liquid lead homogenously and rapidly to produce a protective oxide layer. In this study, the impact of the magnetic force on oxygen transfer in a rectangular container is studied using the lattice Boltzmann method (LBM). Three different Schmidt numbers (Sc) and Hartmann numbers (Ha) have been simulated in this study. Some useful results are obtained such as an adverse effect was found that heat/mass transfer rates are decreased when Ha number is increased. In addition, the existence of an applied magnetic field has caused a significant increase in the required time to reach desired oxygen concentration and needs to be controlled in operation to have a faster distribution of the oxygen in the domain.