Discounting rare but possible mutations the following holds true. Parents blood groups A and A, A and B, A and O, B and B, B and O, O and O can produce type O blood in their children. The rhesus factor (Rhesus positive/rhesus negative) depends on the rhesus pairing in the parents as follows: Father rhesus positive, mother rhesus positive or rhesus negative = rhesus positive or rhesus negative Father rhesus negative, mother rhesus positive = rhesus positive or rhesus negative Father rhesus negative, mother rhesus negative = rhesus negative Example: A rhesus positive father and rhesus positive mother of blood type A and O respectively could produce an O positive child, as could a rhesus positive father and rhesus negative mother both of blood group O. Follow the link to an excellent site.