In a large skillet over medium heat, sauté the pancetta in the oil, stirring often, until it’s crisp and browned, about 8 minutes. Transfer the pancetta to a plate lined with paper towels. Raise the heat to medium high, add the onion and chile flakes, season with salt, and cook, stirring often, until the onion is soft and translucent, 6 to 7 minutes.
Put the pasta in the water. Add the puréed tomatoes to the skillet and cook to thicken the sauce slightly, about 5 minutes. Stir in the oregano and season generously with salt and pepper to taste.
Finish cooking the pasta until it’s just tender, about 10 minutes total. Drain and add to the sauce. Toss well with the pancetta and 1/2 cup of the Pecorino. Serve immediately, topped with a spoonful of the sauce from the bottom of the pan and a sprinkling of the remaining Pecorino.