According to the blood type calculator, an AB and O couple can only have children who are either A or B.
The genotypes for both would be AB and OO, so with the O being recessive, only either the A or B can be passed on from the AB partner and the O would not show up in the phenotype.
So how come several AB and O couples have found me to alert me that they have children who are both phenotype AB and O?
The allele which they carry is called cisAB where both, the A and the B is being encoded into one allele and in their case their phenotype is in fact AB while their genotype would be cisAB/O.
In that case, a cisAB/O and O/O couple can only produce cisAB/O and O/O children meaning their offspring´s phenotype can only be AB or O, but never A or B.