I was doing OPT as a part-time from June 1st, 2007 to May 31st, 2008.
With my condition, I didn't realize I didn't need to pay both Social Security Tax and medicare.

This April, 2008, I did my tax-return for a half year (June 1st to December 31st, 2007)
Is it possible for me to get the money back for entire year now?

Thank you for your kind advice!

1. For 2008, you should ask your employer to refund you the FICA (ss and Medicare) taxes.
2. For 2007, you will still need to write to your employer for refund. You can send them an email. Then you need to file Form 843 with IRS.
Read Your U.S. Tax Return: Social Security and Medicare (FICA) Taxes for Non-resident Exempt Individual (http://taxipay.blogspot.com/2008/03/non-resident-exempt-individual-social.html)

When you file the Form 843, you will need a letter from your employer stating that they refuse to refund the FICA taxes.

The IRS requires this because it is preferred that the refund be given FIRST to the employer (who, after all, actually paid the tax).