I too am curious as to how MukatA determined you had "tax treaty benefits on OPT income" without knowing from which country you originate.
You can determine what your tax treaty benefits are by either accessing the treaty itself (see link below):
Index of /pub/irs-trty/
Or by reviewing IRS Pub 901, which provides a plain synopsis of all of the tax treaties currently in effect.
If your country is not listed in either IRS Pub 901 or at the link above, then your country has NO tax treaty with the U.S.