A man with type AB blood cannot be the father of a child with type O blood. A child with type O blood would have to get the O allele from both its father and mother.
In the ABO blood typing system in humans, three types of alleles, or forms, of the blood type gene exist: A, B and O. Having type O blood is a recessive trait that requires two O alleles. The A and B alleles are codominant, so both are expressed. Each person can inherit only two alleles, one each from both mother and father. A man with AB blood can only pass on either the A or B alleles to his offspring, not an O allele.