In a large bowl, whisk together the olive oil, lime juice, minced garlic, cumin, paprika, oregano, salt, black pepper, and cayenne pepper if using.
Add the chicken thighs to the marinade, ensuring they are well coated. Cover and refrigerate for at least 2 hours, or preferably overnight for maximum flavor.
Preheat your grill to medium-high heat.
Remove the chicken from the marinade and let any excess drip off. Discard the marinade.
Place the chicken on the grill, skin side down, and cook for about 6-8 minutes. 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.