Hemoglobin O (HbO) is a rare type of hemoglobin in which there is a substitution of glutamic acid by lysine as in hemoglobin C, but at different positions. Since the amino acid substitution can occur at different positions of the β-globin chain of the protein, there are several variants. In hemoglobin O-Arab (HbO-Arab) substitution occurs at position 121,[1] while in hemoglobin O-Padova (HbO-Padova) it is at 11 position,[2] and in hemoglobin O Indonesia (HbOIna) it is at 116.[3]
HbO is usually harmless unlike other hemoglobin variants such as HbS and thalassemias, even under combination with these abnormal hemoglobins. Hemoglobin O-Padova is the most severe form and is associated with disease of the RBCs and spleen.