In a large bowl, whisk together olive oil, oregano, garlic powder, salt, and black pepper until well combined. Add the chicken thighs, ensuring they are fully coated, cover and refrigerate for at least 30 minutes.

Heat a large skillet over medium-high heat and add the marinated chicken thighs. Sear for about 5-7 minutes on each side until cooked through, reaching an internal temperature of 165°F.

Prepare the creamy feta tzatziki by combining Greek yogurt, crumbled feta cheese, lemon juice, minced garlic, salt, and pepper in a medium bowl. Stir until smooth.

Once the chicken is cooked, let it rest for a few minutes then slice it. To assemble, place warm pita bread on a plate, spread tzatziki, add chicken slices, top with tomatoes, cucumber, and onion, and roll tightly.