Preheat the oven to 350 degrees F. Prepare the peppers by slicing off the tops and removing the seeds and stems. Spray or grease a 9x13 baking dish with oil. Place the hollow peppers in the dish.
Cook the spaghetti according to package instructions. Spaghetti typically comes in 16 oz packages, so you can choose to make it all and use half or just make half of the package. When the spaghetti is finished, drain it and transfer to a large mixing bowl.
While the spaghetti is cooking, dice the tops of the peppers (to prevent food waste). Heat the olive oil in a large skillet over medium high heat. Add the diced pepper tops and diced onions, and cook for 5 to 7 minutes. Transfer these veggies to the mixing bowl with the spaghetti.
Add the diced tomatoes, oregano, grated parmesan, and shredded mozzarella to the bowl with the spaghetti and sauteed veggies. Stir until everything is well combined.
Using tongs, stuff the bell peppers with equal amounts of the spaghetti mixture. Bake for 35 to 40 minutes or until the peppers become slightly tender. Serve with a sprinkle of cheese on top, and enjoy!