Place steak in a shallow glass baking dish. Rub with oil on each side. Sprinkle with oregano, salt and pepper. Sprinkle orange juice, lime juice, and vinegar over the 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 (or gas grill). Drain meat, reserving marinade. Place steak on grill. Top with orange slices. Occasionally spoon reserved marinade over steaks as they cook. Grill 3-4 minutes on each side, or until mediumrare. Cook longer if desired. Remove orange slices to turn steak. Replace orange slices on top of steak.