It's up to you and whether you mind eating that late or not, and how bad you want to eat there. We eat at Ohana every trip so we know how hard the reservation can be to get. Don't worry about it closing at 10:00, no one expects to leave at that time, it's just the cutoff for reservations. With that said, don't take a chance at being late. I would leave the MK no later than 9:00, maybe a little earlier to be safe. You could enjoy the Poly or maybe even get seated earlier if lucky. We love Ohana after dark, so that's an advantage IMO.
It is an awesome experience, really the only fixed price meal we'll still spend the money on. So if you're okay with eating that late, go for it! (and keep trying for something earlier in case it opens up)