Preheat oven to 425°F. Line a rimmed baking sheet with parchment. In a small bowl, combine Parmesan, oregano, salt, pepper and garlic powder. Add olive oil and stir to form a moist mixture.
Arrange tomato slices on baking sheet and spoon a heaping 1/2 tsp. Parmesan mixture on top of each slice, dividing evenly. Use your fingertips to press into an even layer. Bake until tomatoes are soft and topping begins to brown, about 20 minutes. Serve hot or warm.