I would like to know whether you have to pay FICA taxes while working under OPT. If not, what happens if you have already paid it last tax year, are you able to reclaim the amount. Thank you,

You are EXEMPT from FICA taxes while on OPT because you are on a F-1 visa, but only if you arrived in the U.S. in or after 2003.

You should ASK the employer to refund these improperly-withheld taxes back to you. If they refuse, get it in writing, then file Form 843 to request a refund.