REMOVE seeds from peppers. Bring 1-1/2 qt. (6 cups) water to boil in Dutch oven on medium-high heat. Add green peppers. Reduce heat to medium-low; simmer 5 minutes or until peppers are crisp-tender. Drain.
PREHEAT oven to 350°F. Cook and stir onions and garlic in butter in medium skillet until garlic is tender and fragrant. Stir in tomato sauce, rice, 1 cup of the cheese and the oregano; spoon evenly into pepper halves. Place in 12x8-inch baking dish.
BAKE 20 minutes. Top with remaining 1/2 cup cheese; continue baking until cheese is melted.