Preheat the oven to 375°F (190°C).
In a large skillet, heat olive oil over medium-high heat. Add the onion and garlic and sauté until softened, about 3-4 minutes.
Add the ground turkey to the skillet and cook until browned, breaking it into small crumbles. Drain any excess liquid.
Stir in the cooked rice, half of the tomato sauce, oregano, salt, and pepper. Mix until well combined.
Place the prepared bell peppers upright in a 9x9 inch baking dish. If they are unstable, trim a thin slice off the bottom without creating a hole.
Spoon the turkey mixture into each pepper until full.
Pour the remaining tomato sauce over the top of each stuffed pepper.
Cover the baking dish tightly with aluminum foil and bake for 30 minutes.
Remove the foil, sprinkle cheese over the peppers, and return to the oven for 10-15 minutes until the peppers are tender and the cheese is bubbly.
Let rest for 5 minutes before serving.