The prof says that if not parents have type O, then child will have type O.
The student counters that in his case, not both parents have type O, yet he has type O.
That sounds like the student believes that the professor thinks it is impossible for someone without type o to have a kid with type o. In other words, they think the prof thinks that only those with type o can have a kid with type o. The student mistakes the professors argument, that both type O is sufficient for child with O, for both having type O being a necessary condition for the Chile having type O.
I'm I being clear with this? Weirdly, short questions like this are often the most confusing.