Combine the oil and garlic in a small bowl and rub the mushroom caps on all sides with the mixture. Place the caps, top side down, in a circle on an oiled baking sheet. Season with the salt and pepper. Arrange the cheese, basil, and tomato slices alternately in a circle on top of the mushrooms. Sprinkle with the oregano, if using. Bake at 450 degrees F until the cheese melts, about 3 minutes.