Cook the bacon in a large, deep skillet over medium-high heat, until evenly browned and crispy, about 10 minutes; drain on a paper towel-lined plate. Crumble once cool to the touch.
Beat the cream cheese and mayonnaise together in a bowl until light and fluffy.
Add the Parmesan cheese, most of the crumbled bacon, and most of the green onion into the cream cheese mixture, reserving just enough bacon and green onion for garnish; stir until all ingredients are evenly incorporated. Chill in refrigerator until slightly firm, 1 to 2 hours; shape the mixture into a ball and roll in the reserved bacon and green onions. Return to refrigerator until set, 8 hours or overnight.
