In a mixing bowl, whisk together olive oil, oregano, garlic powder, salt, and pepper. Add chicken thighs and ensure they are coated. Cover and refrigerate for at least 30 minutes, preferably overnight.

Heat a large skillet over medium-high heat. Cook marinated chicken thighs for 5-7 minutes on each side until golden and internal temperature reaches 165°F. Remove from skillet and let rest before slicing.

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

Slice cooked chicken and warm pita bread if desired. Spread tzatziki on the bread, add chicken, and top with tomatoes, cucumber, and red onion. Roll tightly.