In a skillet melt butter and add onion slices. Drizzle with reserved marinade mixture. Saute the onion until slightly brown. Set aside.
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.