In a small bowl, stir together olive oil, steak seasoning, chili powder, oregano and black pepper. Heat grill or broiler to medium hot. Spread rub on both sides of flank steak. Let sit while grill heats up. Once grill is hot, grill meat for 5 minutes; turn over and continue to grill for an additional 3 minutes.
Meanwhile, prepare two packages (4 ounces each) Southwest mashed potatoes as package directs, using 3-3/4 cups water and adding 2 tablespoons unsalted butter at the end. Slice steak; serve alongside potatoes with spicy ketchup or steak sauce on the side. Makes 6 servings.