Combine first 6 ingredients in a shallow dish. Add steak, turning to coat with marinade. Cover and chill 8 hours.
Heat 3 tablespoon oil in a large cast-iron skillet over medium-high heat. Add bell peppers, onions, and oregano. Cook 12 minutes or until tender, stirring often. Add remaining minced garlic, and cook 2 minutes, stirring often. Stir in cilantro. Keep warm.
Warm tortillas according to package directions.
Remove steak from marinade, discarding marinade. Grill steak, covered with grill lid, over medium-high heat (350° to 400°) 5 minutes on each side or to desired degree of doneness. Transfer steak to a cutting board, and let stand 10 minutes. Cut steak diagonally across the grain into thin strips. Serve with tortillas, onions and peppers, and desired toppings.