shmoel – Even if his children would have married Bnei Noach, it is only with Jews that yichus (as to nationhood) is matrilineal. For everybody else it is patrilineal, hence Og’s sons would be Bnei Og, and if they would have married daughters of Bnei Noach their children would also be Bnei Og. Only the daughters’ children would become Bnei Noach by virtue of being the children of male Bnei Noach. Whilst, over time, this would mean that the vast majority would become Bnei Noach, there would always be a few who were ben achar ben from Og and therefore not Bnei Noach. So to say “all would be Bnei Noach” is incorrect. The vast majority, maybe, but not all.
I do, however, agree that it is far from certain that Og had any children post-mabul. At any rate, bizman Avraham Avinu, Og was not married.