In a bowl, whisk together olive oil, lemon juice, oregano, garlic powder, salt, and pepper.
Place chicken breasts in a resealable bag.
Pour marinade over chicken, seal the bag, and refrigerate for at least 30 minutes.
Preheat the grill to medium-high heat.
Remove chicken from the marinade and discard the marinade.
Grill chicken for 6-7 minutes on each side or until cooked through.
Check internal temperature (should reach 165°F).
Remove chicken from the grill and let it rest for 5 minutes.
Slice the chicken and arrange on a serving plate.
Garnish with fresh parsley, olives, and feta cheese if desired.