Cut steak into serving-size pieces; place in a 6-qt. electric pressure cooker. In a large bowl, combine tomato sauce, broth, soup mix, oil, vinegar, oregano, garlic powder and pepper; pour over meat. Scrub and pierce potatoes; place over meat.
Lock lid; close pressure-release valve. Adjust to pressure-cook on high for 15 minutes. Quick-release pressure. Press cancel. A thermometer inserted into beef should read at least 160°. Remove meat and potatoes; keep warm.
For gravy, skim fat from cooking juices; return to pressure cooker. In a small bowl, mix cornstarch and water until smooth; stir into pressure cooker. Select saute setting and adjust for low heat. Simmer, stirring constantly, until thickened, 1-2 minutes. Serve with meat and potatoes.