In a large bowl, whisk together the olive oil, oregano, garlic powder, salt, and pepper until well blended. Add the chicken thighs, ensuring they are fully coated. Cover and marinate for at least 30 minutes or overnight for best results.

Heat a skillet over medium-high heat and add a drizzle of olive oil. Add the marinated chicken thighs and cook for 5-7 minutes on each side until golden brown and cooked through (165°F internal temperature). Remove from heat and let rest.

In a medium bowl, combine the Greek yogurt, feta cheese, lemon juice, minced garlic, salt, and pepper. Stir until smooth and creamy. Set aside to meld flavors.

Slice the rested chicken into strips. On a pita bread, spread tzatziki down the center, then layer with chicken, cucumber, tomatoes, and red onion. Roll the pita tightly and serve immediately.

Serve with extra tzatziki, a Greek salad, or oven-baked fries for a complete meal.