As a local, I'm NEVER able to do advance reservations, since I only know a week or so in advance when we're going to be there. I've actually never had a problem getting many reservations.... but then there's some I don't even try, LOL! I'd never try Le Cellier for dinner, but I do get frequent lunch openings, I've also gotten CRT a few times, for all meals.
For some reason, my family just isn't into the resort restaurants, we prefer the park ones, so I wouldn't know about Ohana, but you can get good reservations if you are a little flexible about timing.
I usually have an idea of which park I'll visit on which day, and then just ask Reservations what's available at my favorites. If I can't get anything we like (VERY rare), then when we get there I ask at guest relations, and am always prepared for CS with favorites in each park, just in case.
Good luck!