In a mixing bowl, combine fresh orange juice, lime juice, minced garlic, olive oil, ground cumin, oregano, salt, black pepper, and red pepper flakes. Whisk to combine.
Place the chicken thighs in a baking dish and pour the marinade over them, ensuring they are well-coated. Cover and refrigerate for at least 1 hour, or overnight for enhanced flavor.
Preheat your grill or oven to medium-high heat (about 375°F/190°C).
Remove the chicken thighs from the marinade, letting excess marinade drip off (discard the marinade).
If grilling, place the chicken thighs skin-side down on the grill. Grill for about 6–8 minutes per side, or until the internal temperature reaches 165°F (75°C).
If baking, place the chicken in a baking dish and bake for 25-30 minutes, or until the internal temperature reaches 165°F (75°C).
Once cooked, remove the chicken from the grill or oven and let it rest for 5 minutes.
Serve the chicken thighs garnished with fresh cilantro if desired.