In a small bowl, whisk together the olive oil, minced garlic, balsamic vinegar, oregano, salt, and pepper. Pour the marinade over the flank steak and let it marinate for at least 30 minutes (or up to 4 hours) in the refrigerator.
Preheat your grill to medium-high heat. Remove the steak from the marinade and grill for 5-6 minutes per side, or until it reaches your desired doneness (130-135°F for medium-rare).
Remove the steak from the grill and let it rest for 5 minutes. Slice the steak thinly against the grain.
In a medium bowl, combine the halved cherry tomatoes, mozzarella balls, and fresh basil leaves.
Drizzle with olive oil and season with salt and pepper. Toss gently to combine.
In a small bowl, whisk together the balsamic vinegar, olive oil, Dijon mustard, honey, salt, and pepper until well combined.
Arrange the sliced flank steak on a large serving platter.
Spoon the Caprese salad over the steak slices.
Drizzle the balsamic dressing over the top and serve immediately.