The Spanish Main Treasure is a term used to refer to the wealth of GOLD and silver that was taken from the Spanish colonies in the Caribbean during the 16th and 17th centuries. The term is derived from the Spanish Main, which was the name given to the area of the Caribbean Sea that was controlled by Spain. The answer to the crossword clue 'ORO' (3 letters) is GOLD, which was one of the main components of the Spanish Main Treasure.