The one false rumor that just won't die is that buses don't start running until 1 hour before the parks open.
Buses start running at 6:30 a.m. Every day. Regardless of what time each individual park opens. And the resort monorail starts running at 7 a.m. So in other words, you'll have no trouble getting to 'Ohana for an 8:15 a.m. ADR.
Since MK won't be open yet, you can even ask if the bus driver could drop you off directly at Polynesian. The bus driver is permitted to do this at his/her discretion ... if the bus is packed, then don't count on it, but if it's relatively empty you could get lucky.
Above, I said you could ask the bus driver. Actually, let me rephrase that. Personally, I wouldn't ask. I'd mention to the bus driver that I have an ADR at 'Ohana and let him/her decide if he/she wants to spread a little pixie dust, rather than putting him/her in the uncomfortable position of maybe having to say no to you.