In a large bowl, combine olive oil, minced garlic, smoked paprika, dried oregano, salt, black pepper, red pepper flakes (if using), lemon juice, and lemon zest. Whisk until well blended.
Add the chicken thighs to the marinade, ensuring they are fully coated. Cover the bowl with plastic wrap 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 remaining marinade.
Place the chicken on the grill, skin-side down. Grill for about 6-8 minutes until the skin is crispy and browned.
Flip the chicken and reduce the heat to medium. Grill for an additional 15-20 minutes, or until the internal temperature reaches 165°F.
Remove the chicken from the grill and let it rest for 5 minutes before serving.