It will be crowded. You can watch the fireworks from a lot of places around Poly. Just go down the steps from Ohana and stand outside the lobby doors and you'll see them. It's just not the same as seeing the fireworks at the Magic Kingdom anymore because the castle itself plays a HUGE role in the show. You can't see anything happening on the castle at all, but the music/fireworks can be seen from all over. Even if it's crowded, you're looking at an angle over most heads anyway so you'll be good. And congrats on the reservation at Ohana on a holiday!