In a large bowl, whisk together the olive oil, orange juice, lime juice, garlic, cumin, oregano, smoked paprika, salt, black pepper, and cayenne pepper (if using) until well combined.
Add the chicken pieces to the marinade, ensuring they are well coated. Cover the bowl with plastic wrap and refrigerate for at least 2 hours, or up to overnight for maximum flavor.
Preheat your grill to medium-high heat.
Remove the chicken from the marinade, allowing any excess to drip off. Discard the marinade.
Place the chicken on the grill, skin side down. Grill for about 6-8 minutes, then flip and continue grilling for another 6-8 minutes, or until the internal temperature reaches 165°F.
Remove the chicken from the grill and let it rest for 5 minutes before serving.
Garnish with fresh cilantro before serving.