Whisk together olive oil, oregano, garlic powder, salt, and pepper in a medium bowl. Add chicken thighs, ensuring full coating. Cover and refrigerate for at least 30 minutes, ideally overnight.

Heat a large skillet over medium-high heat. Remove chicken from marinade, letting excess drip off, and cook for 5-7 minutes each side until golden brown and internal temperature is 165°F. Let rest before slicing.

Combine Greek yogurt, crumbled feta, lemon juice, minced garlic, salt, and pepper in a bowl. Stir until smooth and well mixed.

Warm pita bread in a dry skillet for about a minute on each side. Slice cooked chicken into strips. Spread tzatziki on pita, layer with chicken, cucumber, cherry tomatoes, and red onion.

Roll up each pita firmly and serve immediately with sides like Greek salad or oven-baked fries.