Hi Jessica and welcome to the Disney Parks Moms Panel.
In this instance, I would recommend that you take the bus from
Disney's Caribbean Beach Resort.
As your Advance Dining Reservation at
'Ohana is at 7:30 AM, I would say that you need to be at the bus stop at your Resort hotel by no later than 6:30 AM. You will need to take a bus to the
Magic Kingdom Park and then from there, take either a boat or the monorail across to Disney's Polynesian Village Resort.
Although the official word is that Disney's complimentary transportation is only available one hour prior to Theme Park opening time, Disney are aware that there are many Guests who will have early morning breakfast reservations who rely solely on the complimentary transportation service to get them there on time.
I can tell you that my family have caught Disney buses as early as 6:30 AM many, many times in the past without encountering any problems at all.
In order to put your mind at rest, I would suggest that you speak to a Cast Member at Guest Services at your Resort hotel the day before your dining reservation at 'Ohana for confirmation that complimentary transportation will be available to get you there on time.
Finally, if there are no other Guests on the bus when you board, be sure to let the bus driver know where you are heading to as they may be able to obtain permission to take you directly to Disney's Polynesian Village Resort.
Wishing you a magical trip filled with memories which will last a lifetime!
Emma