Heat oven to 450°. Place a rack in a large roasting pan.
Rub roast with oil, then rub with garlic. Season with 1 tsp of the salt and 1/2 tsp of the pepper. Place roast on rack in pan, rib side down.
Roast at 450° for 20 minutes. Reduce oven temperature to 375° and continue to roast for 70 minutes or until temperature reaches 120°. Tent with foil and let rest 15 minutes.
Meanwhile, make chimichurri. Place parsley, oregano, vinegar, garlic, red pepper flakes and remaining 1/8 tsp each salt and pepper in a blender; pulse until chopped. Gradually add oil and process until combined. Cover until serving.
Thinly slice and serve with chimichurri.