Preheat oven to 350°F (175°C) and line a 9x5-inch loaf pan with parchment paper. Finely dice the onion, mince the garlic, grate the carrot (squeezing out excess moisture), and chop the parsley.
Combine the rolled oats and milk in a medium bowl, stir to submerge all oats, and let soak for 10–15 minutes until softened and porridge-like.
Add the beaten eggs, 1/4 cup ketchup, Worcestershire sauce, and Dijon mustard to the soaked oats and whisk until smooth and uniform.
Stir the diced onion, garlic, grated carrot, parsley, salt, pepper, and Italian seasoning into the oat mixture until evenly combined; let rest 2–3 minutes.
Place ground beef in a large bowl, pour the oat-vegetable mixture over it, and gently fold together with your hands just until combined — do not overmix.
Press the mixture evenly into the prepared loaf pan, filling corners and smoothing the top, or shape freeform on a parchment-lined baking sheet for crispier edges.
Stir together the remaining 1/4 cup ketchup, brown sugar, and 1 tsp Worcestershire sauce until smooth, then brush half over the top of the meatloaf.
Bake for 45 minutes, then brush on the remaining glaze and bake another 15–20 minutes until the internal temperature reaches 160°F (71°C) and the top is caramelized.
Tent loosely with foil and rest for 10–15 minutes before slicing into 1-inch thick pieces; spoon any accumulated juices over the slices before serving.