FeO-containing slag originated from the basic oxygen furnace to the ladle is a major reoxidation source during the following secondary refining. Ladle slag reduction treatment (slag treatment) is one of the common countermeasures adopted to eliminate the steel contamination by FeO reoxidation. The oxygen transfer phenomenon between molten steel and slag was studied during the industrial production of interstitial-free (IF) steel, the measured and calculated oxygen activities in steel were compared, and the Fe–O equilibrium at the slag–molten steel interface was investigated by thermodynamic analysis. With slag treatment, the oxygen potential is higher in the molten steel than in the pre-deoxidation slag; this causes oxygen transfer from the molten steel to the slag, decreasing the efficiency of slag treatment. Based on this, a two-step slag deoxidation process was optimized. The second step further reduced the FeO content. On the other hand, the CaO/Al2O3 (C/A) ratio in the refining slag must be controlled, because it affects the FeO activity and inclusion absorption capacity of the slag. The results suggest that the C/A ratio of 1.2–1.5 and the FeO content of\ 6% are beneficial to refine IF steel.