In a mixing bowl, combine olive oil, minced garlic, dried oregano, ground cumin, salt, and pepper. Whisk until blended. Coat the chicken breasts in this marinade and let sit for at least 30 minutes.

Preheat the grill or skillet over medium-high heat. Cook the marinated chicken breasts for 6-7 minutes on each side until golden brown and cooked through. Let rest for 5 minutes before slicing.

While the chicken is resting, whisk together Greek yogurt and lemon juice in a small bowl until smooth and creamy.

In a separate bowl, combine diced tomatoes, chopped cucumber, diced red onion, sliced Kalamata olives, and crumbled feta cheese. Toss to incorporate all ingredients.

Heat a skillet over medium heat and warm the corn tortillas for about 30 seconds on each side. Keep them warm under a clean cloth.

Assemble the tacos by layering the sliced chicken, fresh salad mixture, and drizzling tangy yogurt sauce over the top. Serve immediately.

Garnish with fresh parsley before serving.