Can O and A make a B baby?

Yes, O (blood group O) and A (blood group A) can have a B baby. Blood group is determined by genes inherited from both parents. In this case, the O parent must be heterozygous (OO), meaning they carry a hidden A gene, and the A parent must be heterozygous (AO), also carrying a hidden O gene. When they have a child, there is a 25% chance the child will inherit the A gene from both parents, resulting in blood group B (AB).