In a mixing bowl, combine olive oil, minced garlic, dried oregano, ground cumin, and salt and pepper to taste. Add the chicken and ensure it's well-coated in the marinade. Let marinate for at least 30 minutes.

Preheat the grill or skillet to medium-high heat. Cook the marinated chicken for 6-7 minutes per side until fully cooked. Let it rest for 5 minutes before slicing.

In a bowl, whisk together Greek yogurt with lemon juice until smooth and creamy. Set aside.

Combine diced tomatoes, diced cucumber, red onion, Kalamata olives, and feta cheese in a separate bowl. Toss gently to mix.

Heat the tortillas for about 30 seconds on each side in a dry pan until pliable.

Assemble each taco by placing grilled chicken on a tortilla, topping with the salad mixture and drizzling with yogurt sauce.

Garnish with chopped parsley and serve immediately.