Why does that one character (I forgot his name) keep on saying "Old Sport"?
It is Gatsby himself who keeps using the expression "Old Sport." He also hints that he attended Oxford in England. At one point, Daisy's husband Tom challenges him on this, and Gatsby explains that during World War I some American officers were given the opportunity to attend some classes at Oxford. Gatsby was an officer in the U.S. Army. He picked up the English habit of calling other men "Old Sport" while attending Oxford, but he denies that he claimed to be an Oxford graduate. The Anglophilic habit Gatsby affects seems to characterize him as a social climber, which helps to explain why he devoted so much of his later life to accumulating money.