In a large bowl, mix together olive oil, minced garlic, dried oregano, ground cumin, salt, and pepper until well combined. Add the boneless chicken breasts, coat thoroughly, cover, and let marinate for at least 30 minutes, or up to overnight.

Preheat grill or skillet over medium-high heat (about 375°F). Cook marinated chicken for 6-7 minutes per side until internal temperature reaches 165°F. Remove and let slice after resting for 5 minutes.

Whisk Greek yogurt and freshly squeezed lemon juice until smooth in a small mixing bowl. Set aside.

Combine diced tomatoes, cucumber, red onion, olives, and feta cheese in a large bowl. Gently toss to mix.

Warm corn tortillas in a skillet for 30 seconds on each side until pliable.

Assemble tacos by placing sliced chicken on warm tortillas, topping with salad mixture, and drizzling yogurt sauce over.

Serve immediately while warm with optional sides.