In a large bowl, whisk together olive oil, oregano, garlic powder, salt, and pepper until well combined. Add the chicken thighs, ensuring they're evenly coated with the marinade. Cover the bowl with plastic wrap and let the chicken marinate in the refrigerator for at least 30 minutes.

Heat a skillet over medium-high heat and add a touch of olive oil. Cook the marinated chicken thighs for 5-7 minutes on each side until golden brown, reaching an internal temperature of 165°F (75°C). Allow the chicken to rest before slicing.

In a mixing bowl, combine Greek yogurt, crumbled feta cheese, lemon juice, minced garlic, salt, and pepper. Stir with a whisk until smooth and creamy. Set aside to let flavors meld.

Slice the cooked chicken into bite-sized strips. On warm pita bread, spread a layer of tzatziki, add the sliced chicken, followed by diced cucumbers, cherry tomatoes, and red onion. Roll the pita to enjoy your gyros immediately.