Combine 2 tablespoons olive oil, balsamic vinegar, dried oregano, 1/2 teaspoon salt, pepper, 1/2 teaspoon red pepper flakes, and half the garlic in a freezer bag. Add the steak, massage to coat, and marinate for 15-30 minutes.
Heat 1 tablespoon of olive oil in a large skillet over medium-high heat. Add the potatoes, seasoned with 1/2 teaspoon of salt and the remaining red pepper flakes. Cook until golden and fork-tender, then remove from the pan.
Add the final tablespoon of olive oil to the same skillet. Sear the marinated steak in batches until browned and cooked to your desired doneness, then remove.
Lower heat to medium, add butter, remaining garlic, rosemary, and fresh oregano to the skillet, stirring until fragrant.
Return the steak and potatoes to the pan, tossing well to coat in the garlic herb butter for 1-2 minutes before serving.