To make the chimichurri sauce, in a glass jar, combine oil, vinegar, onion, garlic, parsley, oregano, red pepper flakes, salt, and black pepper. Shake well.
Pierce steaks with the tip of a knife several times on all Side dish. Place the steaks in a shallow glass or ceramic dish and cover with half of the chimichurri sauce. Cover and refrigerate for several hours or overnight, turning the steaks over once or twice.
When ready to cook preheat a grill to medium-high. Grill steaks for 5 to 7 minutes on each side, while generously brushing with the chimichurri sauce left in the dish the meat marinated in. Cut steaks diagonally across the grain and serve with the remaining chimichurri sauce on the side.