Hello friends,
I am currently working in OPT and been in US for more than 6 years. Also i've been filing regular(Resident) 1040 for tax purposes.
Do i have to pay social security and Medicare taxes on OPT right now or am i exempt ?
What'll happen if i ask my employer not to witheld my social security and medicare taxes ?

Anybody in the similar situation ?

Thanks.

 You are exempt while in OPT. You don't have to pay social security or Medicare. Only after you get an H1B, you will be dual-intent, then only you will have to pay those.

Anybody else ?

I payed both SSN and medicare tax when I was in OPT for Ass. degree because I didn't know the rules. After my bachelors, I didn't pay SSN and medicare Tax while in OPT becuase I found the rules, and asked my employer to stop deducting it from my paychecks. You can ask youe HR, and if they ask you for the source provide them with the information, or ask their lawyer.

 I am in kinda same situation. If i started working, can i work in just regular cash basis for a whole year??

 It will be very helpful if you can please give us the link for the rules.

Thanks.

 If you've been in US for more than 5 yrs, i think you have to pay all your taxes while on OPT.

The link I've provided says "As discussed above, this means that foreign students in F-1, J-1, M-1, Q-1 or Q-2 nonimmigrant status who have been in the United States less than 5 calendar years are still NONRESIDENT ALIENS and are still exempt from social security/Medicare taxes. This exemption also applies to any period in which the foreign student is in "practical training" allowed by USCIS, as long as the foreign student is still a NONRESIDENT ALIEN under the Code. Foreign students in F-1, J-1, M-1, Q-1 or Q-2 nonimmigrant status who have been in the United States more than 5 calendar years are RESIDENT ALIENS and are liable for social security/Medicare taxes (unless they are exempt from FICA under the "student FICA exemption" discussed below)"

http://www.irs.gov/businesses/small/international/article/0,,id=129427,00.html

Giordano,
Thats what my understanding was too. I've been here for more than 6 years and considered as "RESIDENT ALIEN" for tax purposes and i've to pay all the taxes.
So, i think i should just pay the taxes i guess and not mess with IRS.

 http://www.irs.gov/businesses/small/international/article/0,,id=129427,00.html 

It explains it all.