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In numerous Harry Potter books, Cho Chang is described as a year older than Harry. And, in OotP, Harry and the other 5th years are taking their O.W.L.s. Then why did Hermione say in the movie, "And she's so frightened that she'll fail her O.W.L.s because she's so worried about everything else"? She should have completed her O.W.L.s and gotten her results!