Question

A child has a blood type of O. Can the mother of that child have a blood type of AB?

Answer 1

Explanation:

Yes they can. An AB parent can indeed sometimes have an O child. But it is by no means common. In fact it would be fair to say that it is exceedingly rare.

The one exception is in certain Asian groups. Some of these folks have a rare version of the ABO blood type gene called cis-AB. People with this gene version have an AB blood type but can easily have an O child.

Answer 2

It’s not common, but yes it’s possible. Though, you need to get an O allele from both parents to have blood type O, there are a few ways:
-If that parent is cis-AB (that is carrying A and B alleles on the same chromosome) and the other allele is O, the parent will have the alleles (AB) O and may pass the O allele to his/her child.

Source: Jack Smith - Quora