Becoming an FSO is more about your general aptitude, future goals and ambitions than what kind of degree you have. Learning to speak a foreign language(s) would be a good start. More common foreign languages like Spanish, French, Italian and German tend to have lots of applicants with those on their resumes. However languages like Farsi, Chinese, Arabic, etc would make you shine out more-so than others.
When they are looking to fill vacant FSO position they are looking for applicants who can fit that role. Determine what place(s) you would like to work and learn languages there along with any customs and history would be helpful. Also, it would be good advice to keep up with current events locally and remotely. Remember that a FSO personel acts as somewhat of an ambassador of the United States. Avoid situations that could get you in a legal mess and keep your finances in order.
As Kim mentioned earlier, those FSO Exams are hard and many of the questions might make you think what does it have to do with being an FSO. They are testing your aptitude, looking for patterns in how you answer the questions/think, overall rounded-ness (politically and socially), and behavioral insights to name a few.