F-1 students who've been in US for < than 5 yrs are exempt from paying social security taxes, but have to pay federal, state, local ( if its applicable in your state ). There is also a provisin in IRS rule that allows you to reduce your federal taxex, by deducting expenses like, food, housing, and transportation, while you're in OPT. but you need to let your employer know about not dedcuting the FICA tax ( SS) from your paycheck sicne your emploeyr might not be aware of this . tell them too lik into IRS Tax publication 519.