Whisk together olive oil, lemon juice, minced garlic, oregano, salt, and pepper in a small bowl to create the marinade.
Place the chicken fillets in a shallow dish or resealable bag and coat thoroughly with the marinade mixture.
Allow chicken to marinate for 15 minutes at room temperature or up to 2 hours in the refrigerator.
Preheat the grill to medium-high heat, reaching a temperature of approximately 200°C (400°F).
Clean and lightly oil the grill grates to ensure the chicken does not stick.
Place chicken fillets on the grill and cook for 5 to 6 minutes per side.
Use a meat thermometer to confirm the internal temperature has reached 74°C (165°F).
Remove the fillets from the grill and let them rest for 5 minutes to allow juices to redistribute before serving.