Can a blood type AB/O couple have AB and O children?

The claim that blood type AB people cannot have blood type O offspring and vice versa is false.

In detail:

When one parent carries a Cis AB allele, the other allele can be any of O, A or B and the phenotype of this parent is always AB, but the children will inherit either the AB or the other allele from this parent.

1. If the other parent is O phenotype (OO genotype) there are three possible scenarios for the blood group of children of a Cis AB carrier (and a 4th very unlikely scenario):
   1. The second allele is O: children are either AB or O
   2. Second allele is A: Children are either AB or A
   3. Second allele is B: Children are either AB or B
   4. A very rare 4th possibility exists: if the other allele is also Cis AB then the children will be always AB irrespective whatever the other parent is, because they will have one cis AB allele from this parent.
2. If the other parent is type A, depending on whether this parent is genotypically AA or AO and what the other allele is in the Cis Ab carrying parent, the following scenarios are possible:
   1. Other parent is AO and second allele is O: The children are either AB or A or O
   2. Other parent is AA and the second allele is O: The children are either AB or A
   3. Other parent is AO and second allele is A: The children are either AB or A
   4. Other parent is AA and the second allele is A: The children are either AB or A
   5. Other parent is AO and the second allele is B: The children are either AB or B
   6. Other parent is AA and the second allele is B: The children are always AB
   7. Rare situation: If the other allele is also cis AB:The children are always AB
3. Likewise, there will be similar scenarios for the other parent being type B:
   1. Other parent is BO and second allele is O: The children are either AB or B or O
   2. Other parent is BO and second allele is A: The children are either AB or A
   3. Other parent is BO and the second allele is B: The children are always AB or B
   4. Other parent is BB and the second allele is B: The children are either AB or B
   5. Other parent is BB and the second allele is O: The children are either AB or B
   6. Other parent is BB and the second allele is A: The children are always AB
   7. Rare situation: If the other allele is also cis AB:The children are always AB

(Caution: ABO inheritance is generally derived assuming the children are not the very rare Bombay phenotype which would require both parents to be carriers of it.)

So don’t freak out if you are blood type O and your dad is AB. Years ago every doctor would have claimed that it is impossible, but now we know it isn’t.

Blood types are simply not reliable ways to exclude paternity in general. More reasons here:
4 major reasons why blood type tests can show different results