Mix together your balsamic vinegar, garlic, oregano and salt in a gallon sized ziplock bag.
Add your flank steak into the bag and sealing, removing all the air.
Massage the marinade into your meat for about 2 minutes.
Let marinate for about 45 minutes in the fridge.
Remove from fridge and let rest on the counter for 15-20 minutes.
Cook on a grill preheated to about 450 degrees.
Cook for 8-10 minutes, flipping half way through.
Let rest on a cutting board for 5 minutes.
Slice across the grain as thick of pieces as you'd like.
Serve with roasted veggies or potatoes.