Trim fat from steak, if necessary. Place steak in a resealable plastic bag. For marinade mixture, stir together oil, lime juice, fajita seasoning and oregano. Reserve 2 tbsp of the marinade and pour the rest of the marinade over meat and seal the bag. Refrigerate for about 4 to 24 hours, turning occasionally.
In a skillet melt butter and add onion slices. Drizzle with reserved marinade mixture. Saute the onion until slightly brown. Set aside.
Drain steak and discard marinade. Grill steak 14 to 18 minutes for medium-rare or to desired doneness, turning once.
Thinley slice meat. Serve with onions and sliced mango in warm tortillas.