Prepare the chimichurri by whisking parsley, garlic, oregano, olive oil, vinegar, red pepper flakes, salt, and pepper in a bowl.
Reserve 0.25 cup of the chimichurri sauce for serving and set aside.
Coat the chicken thighs with the remaining chimichurri in a large bowl or plastic bag and marinate for at least 30 minutes in the refrigerator.
Preheat the grill to medium-high heat, approximately 400 degrees Fahrenheit.
Place chicken on the grill skin-side down and cook for 6 to 8 minutes until skin is golden and crispy.
Flip the chicken and continue grilling for 8 to 10 minutes until the internal temperature reaches 165 degrees Fahrenheit.
Remove chicken from the grill and let rest for 5 minutes.
Serve the grilled chicken topped with the reserved chimichurri sauce.