Gatsby keeps using the term "Old sport" which he claims he picked up at Oxford. He wants people to think that he is an Oxford man. His business associate Meyer Wolfshiem tells everybody, including Nick, that Gatsby is an Oggsford man. Late in the novel Tom Buchanan confronts him about his so-called Oxford background when Tom, Daisy, Nick, and Jordan Baker are spending a hot afternoon in a suite at the Plaza Hotel in Manhattan. Gatsby tells him:
"It was in nineteen-nineteen. I only stayed five months. That's why I can't really call myself an Oxford man....It was an opportunity they gave to some of the officers after the Armistice," he continued. "We could go to any of the universities in England or France."
This appears to be the straight truth, since Tom does not question him further, and Nick writes:
I wanted to get up and slap him on the back. I had one of those renewals of complete faith in him that I'd experienced before.