Father has o positive blood mother has o positive rhfactor blood child was born with b positive blood how can this be?

Blood type genetics: Abo blood type refers to antigens present on red blood cells. From each parent you inherit an a antigen, a b antigen, or no antigen. People inheriting an a and an a are type a. Inheriting an a from one parent and nothing from the other is also type a. Inheriting an a and a b creates type ab. People who are type o have no antigens. If each parent is type o, the children must be type o.