'Ohana will probably be about 90 minutes, especially if you have young children who want to participate in the coconut races and other activities. Travel time, bag check and getting through the tapstile, and getting wristbanded will probably be about 45 minutes. Most likely you'll be getting back to the park between 7:00-7:30 if your reservation at 'Ohana is at opening which is usually 5:00. I would not make a reservation for any later.
On the other hand, you can get into the MK with just your party ticket at 4:00. If you want to do some rides that won't be open during the party or want to see some rare characters, the lines start around 5:30-6:00 so that you don't waste party time in a long line. Many will have a counter service meal or an early TS reservation in the park so that they're finished before the start of the party.
We've done it both ways, but usually we'll have an early dinner in the park so that we don't miss any party time.