Place steak in a shallow glass baking dish. Rub oil on each side of steak. Sprinkle with oregano, salt and pepper. Sprinkle orange juice, lime juice and vinegar over steak. Cover and refrigerate overnight for best flavor or several hours, turning occasionally. To cook, bring meat to room temperature. Prepare and preheat charcoal grill. Drain meat, reserving marinade. Place steak on grill over hot coals. Top with orange slices. Occasionally spoon reserved marinade over steak as they cook. Grill 3 to 4 minutes on each side, or until medium-rare. Cook longer, if desired. Remove orange slices to turn steak. Replace orange slices on top of steak. Can slice it thinly for a taco filling.