Preheat the oven to 375 degrees. Cut the tops off each bell pepper and set them aside. Use a small knife to carve simple jack-o'-lantern faces into the sides of the peppers.
Heat the olive oil in a large skillet over medium heat. Add the onion and garlic and cook for 3 minutes until softened.
Add the ground turkey to the skillet. Cook, breaking it up with a spoon, until no longer pink, about 6 minutes.
Stir in the cooked rice, marinara sauce, oregano, salt, and black pepper. Cook for 2 minutes until everything is heated through and well combined.
Stand the carved peppers upright in a baking dish. Spoon the turkey mixture evenly into each pepper. Replace the pepper tops if desired.
Sprinkle the mozzarella cheese over the filling. Bake for 25 minutes until the peppers are tender and the cheese has melted.