Cook the spaghetti according to the package instructions until al dente.
While the pasta cooks, heat a large skillet over medium heat and add the sausage.
Add the sliced onions, bell peppers, and garlic to the skillet.
Continue to cook until the vegetables are softened, about 5-7 minutes.
Stir in the crushed tomatoes, oregano, and red pepper flakes.
Let the sauce simmer for 10-15 minutes, stirring occasionally.
Once the pasta is cooked, drain it and add it to the skillet.
Season with salt and pepper to taste, and toss to combine.
Serve immediately, garnished with fresh basil leaves.