Pulse tomatoes with their juice in a food processor until chopped. Simmer tomatoes with garlic, oregano, and 1/2 teaspoon salt in a 3-quart heavy saucepan, uncovered, stirring occasionally, until thickened, about 20 minutes.
While sauce is simmering, stir together bread crumbs and milk in a large bowl and let stand 5 minutes. Add meat, onion, 3 tablespoons parsley, red pepper flakes, and remaining teaspoon salt and blend with your hands until just combined well (do not overmix). Form 2-tablespoon amounts into meatballs (about 20).
Heat oil in a 12-inch heavy skillet over high heat until hot but not smoking, then sauté meatballs in 2 batches, turning occasionally, until well browned, about 5 minutes. Transfer as browned with a slotted spoon to tomato sauce.
Simmer meatballs, covered, stirring occasionally, until cooked through, about 5 minutes. Serve sprinkled with remaining 2 tablespoons parsley.
