Preheat the oven to 425 degrees F. Heat the olive oil and sliced garlic in a large pot or Dutch oven over medium-high heat. Cook, stirring, until the garlic is lightly golden, 2 to 3 minutes. Add the tomatoes, 1 teaspoon salt, a few grinds of pepper and 2 teaspoons oregano. Bring to a simmer, then reduce the heat and simmer gently, stirring occasionally, until the sauce thickens slightly, 15 to 18 minutes.
Meanwhile, make the meatballs: Combine the beef, parmesan, breadcrumbs, parsley, milk, eggs, grated garlic, 2 teaspoons salt, a few grinds of pepper and the remaining 1 teaspoon oregano in a large bowl. Mix well with your hands. Form into thirty 1 1/2-inch meatballs and divide between 2 baking sheets. Bake until firm and lightly browned, 12 to 15 minutes.
Carefully remove the meatballs with a spatula and drain briefly on paper towels, then transfer to the sauce. Simmer until tender, 15 to 20 minutes.
Transfer the meatballs and sauce to a slow cooker and keep on warm.