1. In a mixing bowl, combine olive oil, oregano, garlic powder, paprika, salt, and black pepper. Add chicken slices and mix well to coat. Let it marinate for at least 15 minutes.
2. Preheat the grill to medium-high heat. Grill the chicken for about 5-7 minutes on each side, until fully cooked.
3. In another bowl, combine Greek yogurt, lemon juice, dill, minced garlic, grated cucumber, and a pinch of salt to make the tzatziki sauce.
4. Serve the grilled chicken on pita bread with tzatziki sauce and your favorite toppings like tomatoes, onions, and lettuce.