Cook the spaghetti according to the package instructions until al dente. Drain and set aside.
In a large skillet or saucepan, heat the olive oil over medium heat. Add the onion and sauté until softened and translucent, about 5 minutes. Add the garlic and sauté for another minute, or until fragrant.
Add the crushed tomatoes, oregano, basil, red pepper flakes, salt, and pepper to the pan. Bring the sauce to a simmer and let it cook for 30-45 minutes, stirring occasionally.
Add the heavy cream to the sauce and stir to combine. Let the sauce simmer for another 10 minutes.
Add the cooked spaghetti to the pan and toss with the sauce until the pasta is well coated. Cook for another 2-3 minutes, or until the pasta is heated through.
Add the Parmesan cheese to the pan and toss with the pasta until the cheese is melted and the sauce is creamy.
Serve the Pasta Napoletana hot, garnished with fresh basil leaves.