Impossible:a) Harry is the best student in DADA (even better than Hermione), his exam went pretty well, and the examiner was very impressed with Harry's Patronus.b) If Harry doesn't get an OWL in DADA, he has no chance to become a Auror; so he will have no reason to take Potions in sixth and seventh year. And he *does* take Potions (the deLuxe edition of HPatHBP has a book of Advanced Potion Making in the cover).
(I figure that just means if it turns out to be true, I look good ;) )What I'm going by here is the fact that JKR has a tendency to make practically anything that the characters are *sure* turn out to be false. And she made a point to mention that Harry was confident he had gotten an 'O' grade.I still think he's going to be aiming to be an Auror - remember McGonagall saying she'd help him? If he does turn out to have failed, I'm sure she can arrange for him to re-sit the exam.