Preheat the oven to 450 F (230 C). Cut the tomatoes lengthwise into approximately 1/3-inch (1 cm) slices. Place them on a baking sheet. Top with Parmesan, oregano, salt and pepper (season according to your taste). Drizzle with some olive oil and bake until tomatoes are tender and the cheese is melted, for about 10-15 minutes.