You can ask for a window table, but it is likely you will be told that the tables are being offered as they open up, and you'll get whatever becomes available. Everyone wants a window table, and there are actually only a very small number of tables that have a view of Wishes that won't be blocked by foliage.
You are likely to still be eating at 9pm. Just do what everyone else does when Wishes starts: get up and walk over to the windows and watch from there. But, be polite to the people who have the window tables and don't walk in front of them, or allow your children to walk in front of them.
If the stars align and you get in early and finish your meal, you can take the 2 minute walk to the Poly's beach and watch from there. It's actually a great view, and they pipe in the music.
Or, you can ditch 'Ohana, eat elsewhere, and see Wishes from inside the park.
If you've never seen Wishes, the best place, by far, is in the square in front of the castle. It'll be happening above you, and is incredible from there. I'd find somewhere else to eat in this case.
If you've seen Wishes before, I'd keep 'Ohana and play it by ear.