Preheat oven to 350 degrees.
In a large bowl, crumble the hamburger into small pieces, then add the oats, milk, ketchup, Worcestershire sauce, beaten egg, onion, chopped parsley, salt, garlic powder, and black pepper. Work everything together with clean hands until well combined.
In a separate dish, prepare the meatloaf glaze by mixing the ketchup, white vinegar, and brown sugar.
Line the loaf pan with aluminum foil, then spread a smear of glaze on the bottom of the pan and add the meatloaf mixture. Press the meat into the pan to form a loaf (alternatively, you can form the meat into a loaf shape on a rimmed baking sheet). Paint the top of the meatloaf with the remaining glaze.
Set the meatloaf in the oven and bake for 1 hour, or until the internal temperature of the meatloaf reaches 160 degrees when tested with a meat thermometer.
Remove the meatloaf from the oven, let rest five minutes, then slice and serve.