Preheat oven to 350°F; gently mix beef, oats, egg, ¼ cup ketchup, onion, garlic, salt, and pepper by hand in a large bowl just until combined. Do not overmix.
Shape into a loaf in a greased 9x5-inch loaf pan; spread remaining ¼ cup ketchup over top.
Bake 1 hour or until center reaches 160°F; rest 5 minutes before slicing.
Meanwhile, cover potatoes with cold salted water in a large pot; boil 15-20 minutes until fork-tender.
Drain potatoes; return to pot and mash with butter and milk until smooth and creamy. Season with salt and pepper.
Slice meatloaf; serve with mashed potatoes and pan drippings drizzled over for extra flavor.