Bring a large pot of salted water to a boil. Add the rigatoni and cook according to package instructions until al dente. Drain and set aside, reserving 1/2 cup of pasta water.

In a large skillet, heat the olive oil over medium heat. Add the Italian sausage and cook, breaking it apart with a spoon, until browned and cooked through, about 5-7 minutes.

Add the diced onion to the skillet and cook until softened, about 3-4 minutes. Stir in the minced garlic, oregano, and red pepper flakes (if using) and cook for an additional 1-2 minutes until fragrant.

Pour in the heavy cream and bring to a simmer. Stir in the Parmesan cheese until melted and the sauce is creamy. Season with salt and pepper to taste.

Add the cooked rigatoni to the skillet, tossing to coat the pasta in the sauce. If the sauce is too thick, add some reserved pasta water until desired consistency is reached.

Serve immediately, garnished with chopped parsley and additional Parmesan cheese if desired.