Preheat oven to 375°F (190°C).
Cook white rice according to package instructions; set aside.
Cut tops off bell peppers and remove seeds and membranes. Set aside.
Heat olive oil in a skillet over medium heat. Add chopped onion and garlic; sauté until softened, about 3-4 minutes.
Add ground beef to the skillet. Cook until browned and cooked through, breaking it up with a spoon.
Stir in cooked rice, tomato sauce, salt, black pepper, and dried oregano. Cook for another 2 minutes to combine flavors.
Stuff each bell pepper with the beef and rice mixture, filling them completely.
Place the stuffed peppers upright in a baking dish. Cover loosely with foil.
Bake for 30 minutes. Remove foil, sprinkle mozzarella cheese on top of each pepper, and bake uncovered for an additional 10 minutes until cheese melts and peppers are tender.
Remove from oven and let cool slightly before serving.