In a mixing bowl, whisk together soy sauce, olive oil, red wine vinegar, garlic, oregano, and black pepper.
Place the flank steak in a resealable plastic bag and pour the marinade over the steak. Seal the bag and refrigerate for at least 2 hours, or overnight for best results.
Preheat the grill to medium-high heat. Remove the steak from the marinade and discard the marinade.
Grill the steak for 5-7 minutes per side, or until desired doneness. Let the steak rest for 5 minutes before slicing against the grain.
Notes:
For best results, let the steak marinate overnight.