Can A positive father and B mother can give O baby?

Yes, it's possible.

Blood group O is the most common blood type in humans, and it can be inherited from either A or B parents, as well as from O parents. When an A parent and a B parent have a child, the child can have either A or B blood type, or possibly AB if both parents are heterozygous for their respective blood group (i.e. have an A allele and an O allele, or a B allele and an O allele).

In this case, if both the father (who is A positive) and the mother (who is B) are heterozygous for their blood groups, meaning that they each have one A allele and one O allele, then it's possible for them to have a child with blood group O. This is because, in this scenario, there is a 25% chance for each child to inherit the O allele from both the father and the mother.

Blood type is determined by the presence of specific proteins, called antigens, on the surface of red blood cells. A positive indicates the presence of the Rhesus factor (Rh) on the surface of red blood cells, while A negative indicates the absence of Rh factor. In the case of the Rh factor, an individual can be either Rh positive (Rh+) or Rh negative (Rh-), depending on whether or not they have inherited the Rh antigen gene.