MIX onion, steak sauce, oregano and garlic. Place steak in shallow dish or resealable plastic bag. Add steak sauce mixture; turn over steak to evenly coat both sides. Cover.
REFRIGERATE 1 hour to marinate, turning occasionally.
PREHEAT grill to medium-high heat. Remove steak from marinade; discard marinade. Grill steak 4 to 6 minutes on each side or until cooked through.