Hi, Lynne! I think you should be fine to get to the party. The party begins at 7, and really, you can enter any time after that time. You'll want to make sure you're there for the parades and the fireworks, but even then, as long as you make your way into the Magic Kingdom before 8, you will be fine.
I'd say you're in a great place if you want to be in the Magic Kingdom right as the party starts. You can enjoy your dinner, then hop onto the monorail to head into the Magic Kingdom for Mickey's Very Merry Christmas party. I always allow at least an hour to an hour and a half for dinner at 'Ohana, depending on how hungry your family is! That should give you a comfortable lead time to get into the Magic Kingdom right on time!
I hope you enjoy a fabulous dinner at 'Ohana, and have a wonderful time celebrating with Mickey at the Very Merry Christmas Party!