Abstract: The equilibrium reaction between CaO—Al₂O₃—SiO₂—MgO slag and 28MnCr5 molten steel was calculated to obtain the suitable slag composition which is effective for decreasing the oxygen content in molten steel. The dissolved oxygen content O in molten steel under different top slag conditions was calculated using a thermodynamic model and was measured using an electromotive force method in slag–steel equilibrium experiments at 1873 K. The relations among O, the total oxygen content (T.O), and the composition of the slag were investigated. The experimental results show that both O and T.O decrease with decreasing SiO₂ content of the slag and exhibit different trends with the changes in the CaO/Al₂O₃ mass ratio of the slag. Increasing the CaO/Al₂O₃ mass ratio results in a decrease in O and an increase in T.O. To ensure that T.O ≤ 20 ppm and O ≤ 10 ppm, the SiO₂ content should be controlled to <5wt%, and the CaO/Al₂O₃ mass ratio should be in the range from 1.2 to 1.6.