The major that will be the most enjoyable for you as that's likely the one you will get the highest grades in! The selection is very much based on your OPA (along with an application package for OSI). The higher you are, the better the chance. Best way is to do your very best. After that, it's up to the boards.
My friend here with me at RAND was chosen for OSI, but he had to turn it down to come to RAND because they wouldn't hold a spot for him for 3 years. He was prior enlisted, in the scholar's program, high grades (top 10%) and a Behavioral Sciences major. Also did some great community service stuff.