In a sealable jar, bag or small mixing bowl, combine olive oil, lemon juice, onion powder, garlic, salt and pepper. Combine. Add chicken and cover with marinade. Refrigerate covered for 30 minutes or up to 24 hours.
Preheat grill to medium high heat. Place chicken breasts on the grill. Close lid and grill covered 6 min. Rotate chicken and grill 5-7 min when juices run clear and internal temperature has reached 160°F.