Actually.....the buses start running at 6:30, sometimes a bit before that. THe monorail starts about 7am, and the boats are usually running an hour or so before the park opens to resort guests.
For an 8:10am ADR at 'Ohana, I would be out at the MK bus stop at Pop by 7. These early buses run a bit less frequently than regular buses do. When you get to MK, you can take the monorail over to the Polynesian.
Or....when you get on the bus at Pop, ask the driver which is quicker to the Polynesian...boat or monorail. The driver may very nicely offer to drop you right at the Polynesian after he leaves MK...doesn't hurt to ask the question.