Begin by preheating the oven to 350°F (175°C). Coat a 9×13-inch baking dish with non-stick cooking spray.
Clean and peel the Vidalia sweet onions. Slice them sideways into rounds about 1/2 inch thick and separate into rings.
Arrange the onion rings in the prepared baking dish, season evenly with salt, garlic powder, and oregano.
Cut the butter into eight even portions and distribute evenly over the onions.
Sprinkle with Italian blend and cheddar cheese, then top with parmesan cheese.
Bake for 35-40 minutes, until the cheese is brown and bubbly. Serve warm.