Combine the onion, ketchup, vinegar, brown sugar, salt, Worcestershire, chili powder, and oregano in a 4- to 6-quart slow cooker. Stir in the ribs. Cook on low heat for 6 hours, or on high for 3 1/2 hours. Using tongs, remove the ribs from the cooker and arrange on a broiler pan. Heat the broiler. Pour the juices from the cooker into a bowl and place in the freezer for 10 minutes to hasten the separation of the fat. Remove and discard the fat. Pour the defatted juices into a saucepan; boil over high heat until reduced by half, about 15 minutes. Broil the ribs about 6 inches from the heat until browned. Serve with the sauce.