Here is brief information posted at Wheaton College's website:
http://www.wheatoncollege.edu/Global...mployment.html
Q. Once I start working, will taxes be taken out of my paycheck?
A. There are three kinds of tax in the U.S.: federal, state, and Social Security/Medicare. You will have federal and state tax taken out of your paycheck. Social Security/Medicare taxes should not be withheld while you are on OPT unless you are considered a resident alien for tax purposes. For further information, please see IRS Publication 519, Page 42, Chapter 8. IRS Publication 519 is available at:
http://www.irs.gov/pub/irs-pdf/p519.pdf
My personal (non-professional) opinion is that if your income was low enough, you may indeed be able to get your income tax payments refunded--especially likely if you began the OPT in the middle of the year or later.