This study sets out to find some model/s that could calculate the closest oxygen activity of molten steel to the measured oxygen activity in an ASEA-SKF ladle furnace. Ten steel heats grade SAE 52100 were chosen, the oxygen activities of the molten steel after vacuum degassing process were measured, then by means of two different equations and one computer programs the theoretical oxygen activities were calculated and compared to the measured results. In order to calculate the activities of Al2O3 in the top slag four different models and two different constants were applied. Both Wagner and Turkdogan's equations were found to be useful. It was seen that increasing the wt% Al from 0 up to 0.05 in molten steel, increasing CaO/Al2O3 ratio in top slag, and reducing equilibrium temperature of slag-steel could contribute to reduction of oxygen activity of molten steel. The scanning electron microscope observations revealed that the main types of observed non-metallic inclusions in these samples were spinels and calcium aluminates and by increasing the CaO content of the inclusions their equivalent circle diameters grew.