Is it possible for a b positive mom and an O dad to have negative baby?

No, it is not possible for a B positive mother and an O father to have a B negative baby.

A person's blood type is determined by the presence or absence of specific antigens on the surface of their red blood cells. The B antigen is responsible for the B blood type, while the O antigen is responsible for the O blood type. A person with B positive blood has both the B antigen and the Rh factor (a protein on the surface of red blood cells), while a person with O blood does not have the B antigen.

When a B positive mother and an O father have a child, the child can only inherit either the B antigen from the mother or the O antigen from the father. Since the father only has the O antigen, the child cannot inherit the B antigen and therefore cannot have B positive blood. The child will either have O positive blood (if they inherit the Rh factor from the mother) or O negative blood (if they do not inherit the Rh factor from the mother).