Extended tummy tuck: what happens to upper thigh/butt skin & can I get OHIP coverage?

I wish you the best luck while recontouring you body after substantial weight loss. Extended abdominoplasties/tummy tucks are typically an abdominoplasty plus posterior body lifts. The extent of surgery is based on your existing skin excess and your health (how much surgery is safe for you). Sometimes, additional areas are included in a single procedure, sometimes not. I would suggest you address these with a board certified plastic surgeon to discuss your options.
As for as insurance coverage; it depends. Every private insurance provider stipulates what is a covered and what is not in their insurance policy. If you call them, they will tell you what is required for coverage. I am not familiar with Ontario's health insurance plan.
Your first step is to seek a consultation and discus how best to proceed. The second step is to see how to make it happen. That involves the cost and the payor. All the best.

Without including pictures for context, is difficult to give you a meaningful answer. A tummy tuck is not considered medically necessary, and is not covered by medical insurance. The only medical coverage for this procedure is skin excision on weight loss patients who have chronic skin infections that cannot be treated without surgery. Muscle tightening is not an necessary medical condition and is not covered by medical insurance.
Medical insurance is a communal effort to help individuals deal with unusually high cost. Medical care. Financial resources from medical insurance companies should be spent in a manner that creates the most good for the members of the plan. Medical insurance should not pay for cosmetic surgical procedures because it takes resources from those who need proper needed medical care.
I suggest having multiple in person, consultations for local board-certified plastic surgeons in your community to get an assessment and get your questions answered properly.
Best,
Mats Hagstrom, MD