I payed both SSN and medicare tax when I was in OPT for Ass. degree because I didn't know the rules. After my bachelors, I didn't pay SSN and medicare Tax while in OPT becuase I found the rules, and asked my employer to stop deducting it from my paychecks. You can ask youe HR, and if they ask you for the source provide them with the information, or ask their lawyer.