The equilibrium reaction between CaO -Al_2O_3-SiO _2-MgO slag and 28 MnC r5 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 SiO_2 content of the slag and exhibit different trends with the changes in the CaO /Al_2O_3 mass ratio of the slag. Increasing the CaO /Al_2O_3 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 SiO _2 content should be controlled to <5wt%, and the CaO /Al_2O_3 mass ratio should be in the range from 1.2 to 1.6. The equilibrium reaction between CaO -Al 2 O 3-SiO 2 -MgO slag and 28 MnC r 5 molten steel was calculated to obtain the suitable slag composition which is effective for reducing 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 (TO), and the composition of the slag were investigated. The experimental results show both both [O] and TO decrease with decreasing SiO 2 content of the slag and exhibit different trends with the changes in the changes in the CaO / Al 2 O 3 mass ratio of the slag. Increasing the CaO / Al 2 O 3 mass ratio results in a decrease in [O] and an increase in TO To ensure that TO ≤ 20 ppm and [O] ≤ 10 ppm, the SiO 2 content should be controlled to <5 wt%, and the CaO / Al 2 O 3 mass ratio should be in the range from 1 . 2 to 1.6.