In a mixing bowl, whisk together balsamic vinegar, olive oil, honey, minced garlic, oregano, salt, and black pepper until well combined.

Place the flank steak in a large resealable plastic bag and pour the marinade over it. Seal the bag and refrigerate for at least 1 hour, preferably overnight.

Preheat the grill to medium-high heat.

Remove the steak from the marinade and discard the marinade. Grill the steak for about 5-7 minutes on each side, or until it reaches an internal temperature of 135°F for medium-rare.

Let the steak rest for 5 minutes before slicing against the grain.