Have you ever done a Punnett Square? This question forces you to make one. If your cross a homozygous type A (AA) and homozygous type O (OO) you should get 100% AO. Understanding genetics, an AO gene means a type A blood. Therefore, it's 100% type A blood (AO).
He/she is talking about genetic disorders. Interesting topic. An oncologist is a cancer specialist so I don't know the specific cancer disorder but I'm sure there is one.