In a medium bowl, mix together the yoghurt, garlic, oregano, salt, & pepper. Reserve a small amount (~ quarter C.) to serve with the chicken. Add the chicken breasts to the bowl with the yoghurt mixture, turning to coat, and allow to marinate for 20-30 minutes. Grill or broil the chicken breasts for a few minutes on each side, sprinkling them with the crumbled feta after turning them the first time. Garnish with chopped dill and olives and serve with the reserved sauce. I always double the recipe the chicken is great cold or in a sandwich. Enjoy, Melissa