Heat the oil in a large saucepan over medium heat. Add the sausage and cook, stirring occasionally, until browned. Transfer to a bowl.
Add the onion to the saucepan and cook, stirring, until coated with the sausage drippings. Reduce the heat slightly and add the garlic, oregano and red pepper flakes. Stir until the mixture has softened, about 5 minutes.
Add the crushed tomatoes and tomato sauce to the onion mixture. Stir until fully combined. Season with salt and pepper and stir in the cooked sausage. Reduce heat to low and gently simmer the sauce, stirring occasionally, until slightly thickened, about 25 minutes.
Meanwhile, bring a large pot of salted water to a boil over high heat. Add the penne and cook until just tender, 8 to 10 minutes. Drain and transfer to a serving bowl. Add the sauce, sprinkle with Parmesan, if using, toss and serve.