Typically, there are two types: type A and type B. An individual is born with either, both, or none of the blood groups.
If the man has Blood types A or O on either chromosome and the woman has Blood types B or O on the other, it means that they both transferred the blood type O to their offspring.
In other words, a mother with blood type B, and a father with blood type A, produce offspring with blood type O if the father’s genotype is either type BB or type BO.
It means both parents have heterozygous blood types.