In a large skillet, heat the oil over medium-high heat. Add onion, bell pepper, celery, and garlic; season with salt and pepper. Cook, stirring frequently, until vegetables are softened, 5-7 minutes. Add the ground beef to the skillet. Cook, breaking up meat with a wooden spoon, until it is no longer pink, 6-8 minutes. Stir tomato sauce, ketchup, and Worcestershire sauce into beef mixture in skillet. Simmer until thickened, stirring occasionally, 6-8 minutes. Season the mixture with more salt and pepper, as desired. Spoon onto buns, and serve immediately.