Cook the pasta according to package instructions until al dente.
While the pasta cooks, heat olive oil in a large skillet over medium heat.
Add the Italian sausage to the skillet, breaking it apart with a spatula. Cook until browned, about 5-7 minutes.
Stir in the sliced onion and cook for another 2-3 minutes until it becomes translucent.
Mix in the sliced bell peppers and minced garlic, cooking for an additional 3-5 minutes.
Pour in the diced tomatoes along with their juices, then stir in the oregano. Bring to a simmer.
Season the sauce with salt and pepper, adjusting to taste.
Once the pasta is cooked, drain it and add it to the skillet. Toss everything together, adding reserved pasta water as needed.
Top with fresh basil leaves before serving.