Score flank steak on both sides; set aside. In a small bowl, whisk together soy sauce, oil, ketchup, garlic, oregano, pepper, and onion.
Place steak in a zip top bag or shallow baking dish. Pour soy sauce mixture over top. Cover or seal and refrigerate for at least 8 hours, or overnight.
Preheat grill to medium high. Remove steak from marinade and grill for 3-4 minutes per side for medium rare. Transfer to a cutting board and let rest for about 5 minutes.
Meanwhile, combine ingredients for Buffalo blue cheese butter in a small bowl. Mix well.
Thinly slice flank steak and top with Buffalo blue cheese butter.