To marinate the steak, combine the oil, lemon juice, garlic, and oregano in a resealable container. Add the steaks to the marinade and turn to coat. Refrigerate for at least 2 hours.
Meanwhile, to make the oregano butter, use a rubber spat­ula to combine the butter, garlic, lemon juice, oregano, and parsley in a small bowl. Season with salt and pepper. Cover and keep at room temperature until ready to serve.
When ready to cook, heat the oven to 400°F.
Remove the tenderloins from the marinade, discarding the marinade. Pat the steaks dry with paper towels. Season the steaks on both sides with salt- but not pepper, as pepper will burn in the skillet. You can add pepper after the steaks have cooked.
Heat a large cast-iron skillet over high heat until very hot. Add the steaks to the pan and sear for 3 minutes on each side. Transfer to the oven and cook for 5 to 7 minutes for very rare (120°F) or 7 to 9 minutes for rare (130°F).
Remove the steaks from the oven and loosely cover the skillet with aluminum foil. Set aside to rest and let the juices redistribute, 2 to 3 minutes. Transfer the steaks to warmed plates and top each steak with 1 tablespoon of the oregano butter. Serve immediately.