In a large mixing bowl, whisk together olive oil, oregano, garlic powder, salt, and pepper. Add chicken thighs and coat evenly. Cover and refrigerate for at least 30 minutes.

Preheat a skillet over medium-high heat. Cook marinated chicken for 5-7 minutes on each side until golden brown and cooked through.

In a separate bowl, combine Greek yogurt, feta cheese, lemon juice, minced garlic, salt, and pepper. Mix until smooth.

Slice cooked chicken into strips. Spread tzatziki on warm pita bread, layer with chicken, cucumber, tomatoes, and onion. Roll tightly.

Serve immediately with extra tzatziki if desired.