In a bowl, mix together olive oil, lemon juice, minced garlic, oregano, salt, and black pepper.

Add the chicken breasts to the marinade, ensuring they are well coated. Cover and refrigerate for at least 30 minutes, or up to 4 hours.

Preheat the grill to medium-high heat.

Remove the chicken from the marinade and discard the marinade.

Place the chicken on the grill and cook for 6-7 minutes on each side, or until the internal temperature reaches 165°F (75°C).

Remove from the grill and let rest for 5 minutes before slicing.