Mom is blood type O and dad is blood type B. There are only two possible scenarios for the child, one is O and the other is B. Of course, we are talking about the ABO blood type and the others are not analyzed. The specific reason is this, mom O blood type can be considered as 2 O, dad is B blood type, can be considered as 1 O and 1 B. Then so the 2 O and O, B together to form, there are only two cases is OO, OO, OB, OB. because this O belongs to the recessive, B belongs to the dominant. So, OO is O blood type and OB is manifested as being B blood type.