How much tax I have to deduct for OPT students. Also, Is the employer will deduct taxes and give it to me or I have to pay if the employer did not deduct it.

Students on OPT (Optional Practical Training) are considered to STILL be students, probably under a F-1 visa.

The F-1 visa status (provided they have not exceeded FIVE years in country) makes them EXEMPT from FICA (Social Security and Medicare) taxes, per IRS pub 519.

However, the OPT students ARE liable for federal, state and local INCOME taxes, which must be withheld as specified by Form W-4 and the state-equivalent forms.

Note that foreign nationals may NOT claim more than ONE allowance on their Form W-4. If they are MARRIED and from Canada, Mexico, Korea, or Indian, they can claim TWO allowances.