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

Place the fish fillets in a shallow dish and pour the marinade over them. Let marinate for at least 15 minutes.

Preheat the grill to medium-high heat. If using a grilling basket, preheat it as well.

Place the marinated fish fillets on the grill. Cook for about 4-5 minutes on each side, or until the fish flakes easily with a fork and reaches an internal temperature of 145°F (63°C).

Remove the fish from the grill and serve immediately, garnished with lemon slices and chopped parsley.