Cut steak into serving-sized pieces; place in a 5-qt. slow cooker. In a large bowl, combine the tomato sauce, soup mix, oil, vinegar, oregano, garlic powder and pepper; pour over meat. Scrub and pierce potatoes; place over meat. Cover and cook on low for 7 to 8 hours or until meat and potatoes are tender.
Remove meat and potatoes; keep warm. For gravy, pour cooking juices into a small saucepan; skim fat. Combine cornstarch and water until smooth; gradually stir into juices. Bring to a boil; cook and stir for 2 minutes or until thickened. Serve with meat and potatoes.