Inclusion formation and its behavior at the early stage of deoxidation and alloying operation has a considerable influence on steel cleanliness. In the present work, steel alloying, such as Mn, Al, FeSi, and FeMn additions to the liquid steel with different oxygen and sulphur content was simulated with a confocal scanning laser microscope combined with a special addition device. The inclusion formation and the inclusion interaction behavior immediately after the alloying and/or deoxidation were observed in situ. The inclusions were characterized based on both the in situ observation and the quenched sample. The effect of the sulphur and oxygen content in liquid iron, as well as that of the deoxidant type on the formation of oxides/oxysulfides is discussed taking consideration of the thermodynamics of the system. The inclusion behavior on the liquid iron surface, i.e., the interaction after its formation, the dissolution during the high temperature iso-thermal holding, and growth during the cooling was investigated. The dissolution of Mn(O,S) inclusions at 1843 K (1570 °C) was found to be driven by Mn diffusion through the inclusion/liquid iron boundary layer, and its growth during cooling was significantly affected by Marangoni flow.