Can that be not your child if he blood type is o when parents having blood type o and ab?

No, a child cannot have blood type O if one parent has blood type AB. The blood type is determined by the combination of alleles inherited from each parent. An individual with blood type AB can only pass on either an A or a B allele, while a parent with blood type O can only pass on an O allele. Therefore, the possible blood types for the child would be A (from A and O) or B (from B and O), but not O.