Hi,
I was on OPT till August 20, and then my visa status was changed immediately to H1B. While on OPT, I was already working for the same employer that I'm working for now on H1b. From the beginning, my employer withheld both SS and Mecidare taxes. I still qualify for NR status this year for tax purposes.
My question: what is the process to get the SS and Medicare taxes back for the period I was on OPT (as I understand, everyone pays SS and Medicare regardless of resident status once you are on H1B)?

Thanks a lot, any tips are appreciated.

You need to ask the employer to refund these taxes.

If that does not work, submit Form 843 for the refund. See IRS Pub 519, page 48.