Earliest you can get for CP breakfast (they start at 8:05). Even if you get something around 8:30, go at 8 and see if you can get seated then! Walking down an almost empty Main St, eating and finishing before RD is the best way to start a day at MK!
For 'Ohana, it depends on what time Wishes happens that night. I'd say try for 2-3 hours before Wishes. There are usually long waits at 'Ohana, so you want to be sure to factor in time to accommodate that is necessary. When I went for dinner, I waited about 35 minutes, and from what I read on the boards, I got off lucky.