Fitzgerald only worked on the treatment of that film. In fact, his only screenplay credit was for Frank Borzage's THREE COMRADES whose dialogue was completely re-written by its producer Joseph L. Mankiewicz. The film's star Margaret Sullanvan complained that she couldn't speak Fitzgerald's dialogue (a common complaint -- Fitzgerald's dialogue almost without exception was discarded because of its awkwardness when spoken. His dialogue reads much better than it performs).
Mankiewicz's re-write led to Fitzgerald's famous letter pleading for his importance as a writer, as pathetic a performance as ever happened. Mankiewicz quipped that he would be remembered as "the swine who rewrote F. Scott Fitzgerald." But thank goodness he did becasue he saved THREE COMRADES which is one of Borzage's greatest films at MGM.
As for A YANK AT OXFORD, looking at its complicated writing credits, my guess would be that it is George Oppenheimer who is responsible for whatever sparkle the dialogue posseses though Jack Conway's direction is only serviceable at best.
Brian