I think I spent about an hour to 1-1/2 hours for my dinner at O'Hana a couple of weeks ago. Some tables do have views of the Magic Kingdom but many do not. You can see the fireworks from the tables near the windows but you won't see anything that's projected on the Castle - you'll need to be in the Magic Kingdom for that. You can go to the beach at the Poly after dinner to see the fireworks.

Chef Mickey's may have a few tables - way in the back - with a vew but most will not. There is a fireworks viewing area on the same floor as Chef Mickey's but at the other end of the building. If you finish up your meal early enough, you could walk to the Magic Kingdom from the Contemporary. From the Poly, you could take the monorail to the Magic Kingdom.

Be aware, that later seatings might mean you won't be seated exactly your ADR time and you may need to wait a while for a table so that could delay you a bit as well.