Set an oven rack 6-inches beneath your broiler. Preheat the broiler. Set the mushrooms on a non-stick baking sheet. Brush each mushroom, on both sides, lightly with olive oil and season with salt and pepper. Broil the mushrooms 3 1/2 to 4 minutes on each side, or until just tender. Remove from the oven and ensure each mushroom is sitting cap-side down.
Spread the top of each mushroom with tomato sauce. Now divide and top with cheese, bell pepper, cherry tomatoes, olives and oregano. Set mushrooms back under the broiler and cook until the cheese is melted and light golden, about 2 minutes. Top with the fresh basil and serve.