In a medium saucepan, heat 1 tablespoon olive oil over medium heat. Cook 1 small diced onion and 2 minced garlic cloves, stirring frequently, until translucent, 2 to 4 minutes.
Add two 28-ounce cans whole tomatoes in juice and 1/4 teaspoon dried oregano. Simmer, stirring occasionally, breaking up tomatoes with a spoon, until thickened, 20 to 30 minutes. Season with coarse salt and ground pepper.