Cook the spaghetti according to package instructions until al dente.
While the pasta cooks, heat olive oil in a large skillet over medium heat.
Add the sliced sausages to the skillet and cook until browned, about 5-7 minutes.
Remove the sausage from the skillet and set aside, leaving the drippings.
In the same skillet, add chopped onion, bell peppers, and garlic; sauté until softened, about 5-6 minutes.
Return the cooked sausage to the skillet, then stir in the can of diced tomatoes and oregano.
Simmer the mixture for about 10 minutes, allowing the flavors to meld.
Add the cooked spaghetti to the skillet, tossing to combine. If the sauce is too thick, add reserved pasta water a tablespoon at a time until desired consistency is reached.
Season the pasta with salt and pepper to taste, then serve hot, garnished with fresh basil if desired.