In a large bowl, mix together the olive oil, oregano, garlic powder, salt, and pepper until well combined. Add the chicken thighs, ensuring they are coated thoroughly in the marinade. Cover the bowl with plastic wrap and let it marinate in the refrigerator for at least 30 minutes, or overnight for maximum flavor.

Heat a skillet over medium-high heat and lightly oil the surface. Once the skillet is hot, add the marinated chicken thighs, cooking for 5-7 minutes on each side until they are golden brown and reach an internal temperature of 165°F (75°C). Remove the chicken from the skillet and allow it to rest before slicing.

In a mixing bowl, combine the Greek yogurt, crumbled feta cheese, fresh lemon juice, minced garlic, salt, and pepper. Stir the mixture until it is smooth and well blended. Adjust the seasoning as needed to achieve the desired flavor. Set aside to let the flavors meld while you assemble your gyros.

Warm the pita bread on a dry skillet or microwave for a few seconds until pliable. Start by spreading a generous dollop of the creamy feta tzatziki on each pita, then layer with sliced chicken, cherry tomatoes, cucumber, and red onion. Roll the pita tightly around the filling, ensuring everything is wrapped securely, and enjoy your Mediterranean Chicken Gyros!