In a large bowl or baking dish, mix ½ cup Greek yogurt, 2 Tablespoons olive oil, juice of 1 lemon ½ teaspoon garlic powder, 1 Tablespoon oregano, ½ teaspoon black pepper, and 2 Tablespoons oyster sauce together well.
Add 4 chicken breasts and cover with creamy mixture. Refrigerate for 2 hours.
Place chicken on preheated grill set to medium high heat.
Sear chicken on both sides, then set on top rack and turn heat off on that burner. Keep the remaining burner at medium heat and allow chicken to finish cooking with the lid down.
Rotate chicken after 10 minutes, and cook until chicken is just 165˚F.
Remove from heat and cover with foil to rest for 5 minutes before serving.