In a mixing bowl, combine olive oil, oregano, garlic powder, salt, and pepper. Add the chicken thighs to the marinade, ensuring they are evenly coated. Cover the bowl with plastic wrap and refrigerate for at least 30 minutes, or ideally overnight.

Heat a large skillet over medium-high heat. Add the marinated chicken thighs, cooking them for about 5-7 minutes on each side until golden brown and cooked through.

In a separate bowl, combine Greek yogurt, crumbled feta cheese, fresh lemon juice, and minced garlic. Stir until smooth and creamy, then season with salt and pepper to taste.

Warm the pita bread, spread a generous amount of creamy feta tzatziki on each pita, then layer with the sliced chicken, followed by fresh toppings. Roll up the pita.