There are two possibilities for a child born to a person with blood type O and blood type AB, one is blood type A and the other is blood type B. It is not possible to have blood type O or blood type AB. Because a person’s blood type is controlled by the genes of the parents, the antigen of a person with type O blood can only carry the O antigen, and the blood type antigen of a person with type AB blood can be either the A antigen or the B antigen. If the O antigen and the A antigen are together, the child will have type A blood. If the O antigen and B antigen are together, the child is a B blood type. Therefore, the only two possibilities for an O blood type and an AB blood type are either A blood type or B blood type, not O blood type and not AB blood type. If there is an AB blood type or O blood type, it is most likely caused by an error in the laboratory blood type.