Heat the oil in a large saucepan over medium heat until shimmering.
Add the onion and 1 teaspoon salt and cook until softened, about 5 minutes.
Add the garlic and cook until fragrant, about 30 seconds.
Stir in the tomatoes with their juice, the oregano, and red pepper flakes.
Simmer until the sauce is slightly thickened, about 15 minutes.
Season with salt and pepper to taste.
