If the man has O blood and the woman is homozygous for A blood, then the man has two O alleles (OO) and the woman has two A alleles (AA). Since O blood is recessive, for a child to have O blood, they would need to inherit an O allele from both parents.
Based on the parents' genotypes (OO and AA), all of their children will inherit one O allele from the father. Therefore, there is a 100% likelihood that a child will have O blood.