Can parent who are O pos have achild with B blood?

Yes. An O positive parent and an O positive parent can have a child with B blood. This is due to the fact that O blood is recessive and the B blood type is dominant. In order to have B positive blood, a person must inherit the B blood type allele from both of their parents. If a person inherits the B allele from one parent and the O allele from the other parent, they will have B positive blood.