Arrange potatoes in a microwave-safe dish; cover tightly with plastic wrap. Cut a 1/2-inch slit in plastic. Microwave at HIGH 8 minutes or until tender; transfer to a bowl. Microwave buttermilk in a microwave-safe cup 30 seconds; pour buttermilk and oil over potatoes. Mash until mostly smooth.
While potatoes cook, heat bacon in a skillet over medium heat; cook until crisp, turning frequently. Transfer bacon to a paper towel-lined plate; cool and crumble. Add chopped white onion and sugar to drippings in pan; cook 5 minutes. Stir bacon, onion, salt, and pepper into potatoes. Sprinkle with chopped green onions. Serve potatoes immediately.