Each biological parent donates one of their two ABO alleles to their child. A mother who is blood type O can only pass an O allele to her son or daughter. A father who is blood type AB could pass either an A or a B allele to his son or daughter. This couple could have children of either blood type A (O from mother and A from father) or blood type B (O from mother and B from father).
Since there are 4 different maternal blood types and 4 different paternal blood types possible, there are 16 differnt combinations to consider when predicting the blood type of children. In the tables below, all 16 possible combinations are shown. If you know the blood type of the mother and father, the possible blood types for their children can be found.