Using an immersion blender, puree the tomatoes, in the can, until smooth; set aside.
Meanwhile, in a large Dutch oven, warm the oil over medium heat.
Add the garlic and onion and sauté, stirring often, until the onions are softened, but have not taken on any color, for 6-8 minutes.
Add the tomatoes. Fill one of the empty cans with ½ cup cold water. Swirl the water to gather any of the residual sauce in the can. Transfer to the other two cans, working one at a time, and repeating the process. Add the water, now almost a tomato sauce of its own, to the Dutch oven.
Add the oregano, salt, pepper, red pepper flakes and basil. Bring to a boil, then immediately reduce the heat and simmer, stirring every 10 to 15 minutes, for 1 hour. Season with additional salt, if necessary.
Notes
If making in advance, divide the sauce into 3 quart containers and freeze for up to 2 months.
If your sauce is too acidic, add up to 1 teaspoon of sugar, adding ¼ teaspoon at a time.