Hi,
I just started my OPT on Oct 21st'2008. I am also working as a full time employee on OPT. I also purchased a used car and I have paid tax on it. I would like to know that is the tax paid on car under OPT refunded back. Also how % of federal tax is deducted from the monthly pay check.

The sales tax you paid on the purchase of the car is a STATE tax which is probably NOT refundable to you. To be sure, you need to contact the state sales tax department, as the rules in each state are different.

As for your tax withholding, your federal and state income tax withhodling depends on the level of your income and how many exemptions you claimed on the W-4 (for the federal income taxes) and the state-equivalent of the W-4 (for the state income taxes).

You are exempt from FICA (Social Security @ 6.2% and Medicare @ 1.45%).

If these taxes are being withheld, you should contact your employer's payroll department and refer them to pages 48-49 of IRS Pub 519, which explains in detail why students on F-1 visas are exempt from FICA taxes. While on OPT, you are STILL considered to be a student. Once your employer "sees the light", they will refund the taxes already withheld.

I am from Texas, so now will I get back my sales tax paid on my car. Since I am on OPT

Agreed; having lived in Texas some years back, I know Texas has provisions that allow for refunds of state sales taxes paid by visiting foreign nationals.

So can I go ahead to ask for my refund... is there any kind of form which I am supposed to fill up... or any other formality..

I know the forms are available at the Mexican border, as Mexican nationals claim the refund for shopping they do in the U.S.

A visit to the Texas tax department website will probably result in a PDF copy of the form you will need to download and mail in, along with a copy of the car sales receipt.