1. Since I'm on OPT, I should not have to pay FICA, medicare and NYC resident taxes. So are these taxes refunded when I file the tax return in the next year, or simply not deducted from my paycheck at all?

2. If I am receiving a signing bonus of a large amount, are taxes withheld as if I'm earning that much on every single paycheck? If not, how do they decide how much to withhold?

Thanks for answering!

1) The FICA taxes should not be withheld at all. If they are and the employer refuses to refund them, you need to file Form 843 to request a refund.

As for the NYC tax, you need to file as a non-resident of NYC to get those taxes back.

2) Withholding on a bonus is usually 30%.