Pour olive oil into a large Dutch oven over medium heat. When the oil is hot, add the garlic, let sizzle for a minute, then add the celery and carrot. Cook and stir until the vegetables are softened, about 5 minutes. Pour in the tomatoes, then slosh out the can with 1 cup hot water and add that as well. Season the sauce with the oregano and pepper flakes. bring to a rapid simmer, and cook until slightly thickened, about 5 minutes.