Add olive oil, garlic and fresh rosemary in a small bowl, stirring to combine.
Place flank steak on a cutting board. Using a fork, pierce the steak on both sides throughout. Evenly coat all sides of steak with rosemary garlic olive oil. Place in resealable bag and let marinate 30 minutes to overnight.
Preheat a grill to high. Grill the flank steak 3 to 5 minutes per side for medium rare. Transfer to a cutting board and let rest 5 to 7 minutes, covering lightly with foil. Once rested, thinly slice against the grain.