Bring a large pot of salted water to a boil. Add the rigatoni pasta and cook according to package instructions until al dente (usually about 10-12 minutes). Drain the pasta and set aside, reserving about ½ cup of pasta water for later.
While the pasta cooks, heat 1 tablespoon of olive oil in a large skillet over medium heat. Add the Italian sausage, breaking it apart with your spoon. Cook until browned and cooked through, about 6-8 minutes. Once cooked, remove the sausage from the skillet and set aside.
In the same skillet, add the diced onion and cook for 3-4 minutes until softened. Add the minced garlic and cook for another 30 seconds until fragrant, being careful not to burn it.
Pour in the crushed tomatoes, then stir in dried oregano, dried basil, red pepper flakes (if using), salt, and pepper. Return the browned sausage to the skillet and mix everything well. Let the sauce simmer gently for 10-15 minutes to allow the flavors to meld.
Add the cooked rigatoni to the skillet with the sauce. If the sauce is too thick, gradually add reserved pasta water until the desired consistency is reached. Stir to coat the pasta evenly.
Lower the heat and dollop the ricotta cheese over the pasta. Sprinkle the shredded mozzarella and grated Parmesan on top. Cover the skillet and let the cheese melt for about 3-5 minutes.
Once the cheese is melted and bubbly, garnish with fresh basil leaves. Serve immediately and enjoy the rich, cheesy goodness of this classic pasta dish.