In a large skillet or saute pan, heat the oil over medium heat. Add the onions and sauté until they start to soften and turn translucent, about 5 minutes.
Add the garlic, oregano, red pepper flakes, salt and pepper and continue cooking until fragrant, about 1 minute.
Pour in the tomatoes and add the Parmesan rind, if using. Simmer for 10-15 minutes until the sauce has thickened.
While the sauce simmers, bring a large pot of salted water to a boil. Cook the rigatoni until al dente according to package instructions. Reserve 1 cup of the pasta cooking water. Drain the pasta.
Remove the Parmesan rind. Stir the rigatoni into the sauce with 1/4 cup of the reserved pasta water. You can add more as necessary to thin out the sauce.
Season to taste with additional salt and pepper as needed.
Top with grated Parmesan and parsley before serving.