Position rack in center of oven; preheat to 350°F. Line a rimmed baking sheet or 13×9″ shallow baking dish with foil. Soak breadcrumbs in milk in a large bowl until ready to use.
Pulse onion, celery, and garlic in a food processor until finely chopped. (Alternatively, you can finely chop by hand.)
Heat oil in a large skillet over medium. Add onion mixture, 1 tsp. salt, and 1/2 tsp. pepper. Cook, stirring occasionally, until softened, 5-7 minutes. Remove from heat, stir in Worcestershire, and scrape into bowl with breadcrumb mixture. Add beef, pork, eggs, 1/4 cup ketchup, and remaining 1 1/4 tsp. salt and 1 tsp. pepper, then mix with your hands to combine.
Form meatloaf mixture into a long, well-packed, approximately 5″-wide loaf shape on prepared baking sheet. Brush top of loaf with 1/4 cup ketchup. Bake meatloaf until an instant-read thermometer inserted into the center registers 155°F, 60–75 minutes. Let rest 15 minutes before slicing. Serve with more ketchup alongside.