In a mixing bowl, combine olive oil, oregano, garlic powder, salt, and pepper to create a flavorful marinade. Add the chicken thighs to the bowl, ensuring each piece is well-coated. Cover and refrigerate for at least 30 minutes.

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

In a clean mixing bowl, combine Greek yogurt, crumbled feta cheese, lemon juice, minced garlic, salt, and pepper. Stir until creamy.

Slice the rested chicken into strips. Warm pita bread in the skillet for about 30 seconds on each side. Spread tzatziki on pita, layer sliced chicken, and top with veggies.

Assemble the gyros tightly and serve immediately with optional sides like Greek salad.