Heat the vegetable oil in a heavy skillet over medium heat. When the oil is hot and shimmering, add the chopped celery and onion; cook for about 5 to 7 minutes, or until the onion is translucent and the celery is tender.
Whisk the egg in a large bowl. Add the ground beef, oats, half of the chili sauce or ketchup, Worcestershire sauce, salt, pepper, the cooked celery and onion mixture, and the water or milk. Blend thoroughly.
Pack the meat mixture in a 9-by-5-inch loaf pan or meatloaf pan, or shape it into a loaf in a shallow baking pan.
Bake the meatloaf in the preheated oven for 1 hour.
Spread the ketchup over the top and bake for about 10 to 15 minutes longer.
Let the meatloaf stand for 10 minutes before serving.