Hi
I am currently on my OPT status and I am required to fill a W-4 form. I am not supposed to be taxed for SSN and Medicare on my current visa status. Could you please tell me how do I state this on my W-4?

You need to write "non-resident alien" on your form W4 (see instructions below).
There is no way to indicate on the W4 that you are not subject to FICA, you need to talk to your payroll/human resources department.
If you are able to claim a tax treaty exemption, you would need to fill out form 8233

Special Instructions for Form W-4 for Nonresident Aliens.

A nonresident alien subject to wage withholding must give the employer a
Completed Form W-4 to enable the employer to calculate how much income tax to withhold. In completing the form, nonresident aliens
Should use the following instructions:
1)
Check only “Single” on line 3 (regardless of actual martial status).
2)
Claim only one (1) withholding allowance on line 5, unless a resident of Canada, Mexico, or South Korea or a U.S.
National.
3)
Write “Nonresident Alien” or “NRA” above the dotted line on line 6.
4)
Do not claim “Exempt” withholding status on line 7.

Thank you for the response. I have informed my HR about my visa status and told them that I should typically not be taxed for the SSN and Medicare but this is what they said "The form indicates that you would like to claim two allowances (line 5) or include two allowances in your tax withholdings. Please see line 7 on the W4 form and complete a new form accordingly". I have no idea what else to fill out in the W-4.

Please guide me on this.

Thanks
You need to write "non-resident alien" on your form W4 (see instructions below).
There is no way to indicate on the W4 that you are not subject to FICA, you need to talk to your payroll/human resources department.
If you are able to claim a tax treaty exemption, you would need to fill out form 8233

Special Instructions for Form W-4 for Nonresident Aliens.

A nonresident alien subject to wage withholding must give the employer a
completed Form W-4 to enable the employer to calculate how much income tax to withhold. In completing the form, nonresident aliens
should use the following instructions:
1)
Check only “Single” on line 3 (regardless of actual martial status).
2)
Claim only one (1) withholding allowance on line 5, unless a resident of Canada, Mexico, or South Korea or a U.S.
national.
3)
Write “Nonresident Alien” or “NRA” above the dotted line on line 6.
4)
Do not claim “Exempt” withholding status on line 7.

As a non-resident alien you should only claim 1 allowance (not 2)
If you are married, you won't be able to file married filing jointly. You will need to file married filing separately.

If the OP is from INDIA, he CAN claim his wife as a dependent on Form 1040 under the U.S.-India Tax Treaty.

That being the case, he would be justified to claim TWO allowances on his W-4 if the employer allows it.

Thank you for all the answers. I am from india but not married.
This means I can claim only one allowance right?
And just stating "NRA" above line 6 enough to let my company know that I am exempt from SSN and Medicare taxes?or do I have to fill additional forms to do this?

Contact the HR department and discuss the FICA (Social Security and Medicare) tax exemptions.

You may need to show them pages 51-57 of IRS Pub 519 to convince them of the exemption.