How were the blood types determined? Could the O have been typed wrong?
In theory, it shouldn’t happen, but there are lots of examples of type inheritances that don’t follow the rules. Most of the time, the blood types have been done poorly, but I know of circumstances where that’s unlikely and the patterns still don’t fit the rules.
It is possible – if parrent with with tipe O hase this becouse he/she luck protien thac carry polysacharide chein that determnate bloode tipe (If I am not wrong it is called Bombei sindrome). So he/she can carry allele B but not blood tipe B.