• In large, shallow bowl, combine oil, oregano, vinegar, and garlic. Add steak and toss to coat. Cover with plastic wrap and marinate for at least 30 minutes or up to 4 hours, flipping steak halfway through. Remove from refrigerator 1 hour before cooking.
• Preheat grill, grill pan, or large cast iron skillet to medium high heat. Season both sides of steak generously with salt and pepper. Let excess marinade drip off steak, then transfer steak to pan. Cook 5-7 minutes per side until golden brown for medium rare.
• Transfer steak to cutting board and allow steak to rest 5 minutes. Slice steak into ½-inch pieces across grain.
• Assemble gyros by stuffing pita halves with lettuce, onions, tomatoes, and steak. Drizzle with tzatziki sauce and serve immediately.

Note: Extended marinating time required.