The effects of preexisting Mn·OSiO2 and FeO inclusions on the size and morphology of Al2O3 were investigated. Two-stage deoxidation with Mn and Si followed by Al and single-stage Al deoxidation at [O] levels of 287 ppm [O], 612 ppm [O], and 1071 ppm [O] were performed. The results showed heterogeneous nucleation and growth of Al2O3 onto MnO·SiO2 and FeO inclusions that were present in the steel before Al deoxidation. The transformation from MnOSiO2 to Al2O3 was slow and controlled by Al diffusion through an Al2O3 reaction product layer but that the conversion of FeO to Al2O3 was relatively fast and likely controlled by Al diffusion in liquid steel. For both deoxidation processes, clustered Al2O3 inclusions were observed. However, reactions involving preexisting FeO also produced dendrites and large spherical inclusions within the inclusion clusters. For same supersaturation level, the average size and area fraction of the spherical Al2O3 produced from reactions with FeO increased with increasing FeO content. Al2O3 inclusions formed from reactions with Mn·OSiO2 inclusions had a smaller area fraction than the preexisting Mn·OSiO2 inclusions.