Could a man with AB blood father an o child?

Yes, it is possible for a man with AB blood type to father a child with O blood type. This can occur if the mother of the child has OO blood type. When a man with AB blood type (whose genotype is IAIB) mates with a woman with OO blood type, the possible blood types of their children are A (IAi), B (IBi), or AB (IAIB). There is no possibility of having a child with O blood type if both parents have either A or B antigens in their blood.