The cooking time for pork depends on the thickness of the meat; roast until an instant-read thermometer inserted in the thickest part registers 155 degrees.
Preheat oven to 450 degrees. Make the barbecue sauce: In a medium bowl, stir together tomato sauce, 1/4 cup water, onion, garlic, sugar, vinegar, mustard, Worcestershire, 1/4 teaspoon salt, and 1/8 teaspoon pepper. Set aside.
Sprinkle 1/2 teaspoon salt and 1/8 teaspoon pepper over pork. In a medium ovenproof skillet, heat oil over medium-high heat. Place pork in skillet, and sear on all sides until golden brown, about 8 minutes total. Remove from heat, pour off excess fat from skillet, and carefully pour some of the barbecue sauce over pork, covering completely.
Transfer skillet to oven. Cook, basting every 15 minutes, until juices run clear, about 45 minutes; meat should register 155 degrees on an instant-read thermometer. Transfer pork to a cutting board; cover and let rest about 10 minutes, then slice thinly and serve with remaining barbecue sauce.