You've extrapolated an untrue statement from it.
AA + OO = all kids AO, having A blood type is true.
BUT a person with A blood type can have an O recessive. Thus AO + OO means chances are half the kids will be AO (A blood type with O recessive) and half the kids will be OO with O blood type.
It is possible for an A blood type parent to have O blood type children.
I was pretty sure I didn't understand it. Now, I know I don't.