In a bowl, combine olive oil, oregano, garlic powder, salt, and pepper.
Add the chicken slices to the marinade and mix well.
Let the chicken marinate for at least 30 minutes.
Heat a skillet over medium-high heat.
Cook the marinated chicken for about 7-10 minutes until golden brown.
Warm the pita breads in a separate pan or microwave.
Spread tzatziki sauce on the pita.
Add cooked chicken on top of the sauce.
Top with diced tomatoes and sliced onions.
Garnish with fresh parsley and fold the pita to serve.