In a medium-sized bowl, whisk together the olive oil, orange juice, ketchup, soy sauce, lemon juice, lemon zest, garlic, and optional chili flakes.
Place the boneless chicken thighs in a shallow dish or a resealable plastic bag.
Pour the marinade over the chicken thighs, ensuring they are evenly coated. Massage the marinade into the chicken to ensure it's well-distributed.
Cover the dish or seal the bag and place it in the refrigerator to marinate for at least 30 minutes, up to 24 hours, for maximum flavor absorption.