In a bowl, whisk together balsamic vinegar, olive oil, minced garlic, oregano, salt, and black pepper.
Place the flank steak in a resealable plastic bag or shallow dish and pour the marinade over it. Seal or cover and refrigerate for at least 2 hours, or up to overnight for more flavor.
Preheat your grill to medium-high heat. Remove the steak from the marinade and discard the marinade.
Grill the flank steak for about 5-7 minutes per side, or until it reaches your desired doneness (135°F for medium-rare).
Remove the steak from the grill and let it rest for 5-10 minutes before slicing against the grain.
In a large bowl, combine the cherry tomatoes, mozzarella, and basil. Toss gently to mix.
Serve the sliced steak topped with the Caprese mixture and drizzle with balsamic glaze.