Curly hair is dominant to straight hair. A couple, each with blood type O and curly hair, have a child with O blood type and straight hair. ?

Blood type only refers to your blood, not your hair.  Yes, curly hair is dominant over straight hair, but you are only counting one gene from each parent and one generation.  People have both dominant and recessive genes, and genetics includes all generations from both sides of your family.  

For instance, if your ancestors had mostly straight hair, your parents could both have a recessive straight hair gene, and you could have straight hair.