Cook rigatoni in salted boiling water until just under al dente; drain and reserve 1/2 cup pasta water.
Heat olive oil in a large skillet over medium-high heat and brown the sausage, breaking it into small crumbles.
Remove sausage with a slotted spoon and set aside, leaving the rendered fat in the skillet.
Sauté diced onion in the fat over medium heat for 5 minutes until soft.
Stir in garlic, oregano, and red pepper flakes; cook for 1 minute until fragrant.
Pour in crushed tomatoes, season with salt and pepper, and simmer on low for 10 minutes.
Stir in the cooked sausage, heavy cream, and Parmesan cheese until smooth.
Toss the drained pasta into the sauce for 2 minutes, adding a splash of pasta water if needed. Garnish with basil.