I think in your case, you will have to pay social security and medicare taxes since you have been in the US more than 5 calendar years.  For those filing taxes as nonresident alien and if they are on OPT, they are exempted for such taxes.

Social Security and Medicare Tax 

As a nonresident (1040NR), if you are self-employed, you must pay income tax, but not self-employment tax (i.e., social security tax) on the income.  Residents (1040) must pay social security tax.

A nonresident alien employee (filing Form 1040NR) temporarily in the United States on an "F-1," "J-1," "M-1," or "Q-1" visa is not subject to social security and Medicare taxes if one of the following apply:

1.   The pay is for services performed to carry out the purpose for which the alien was admitted to the United States.  

2.   You are working under Curricular Practical Training and Optional Practical Training, on or off campus, as long as the employment is authorized by the Immigration and Naturalization Service.   

3.   If you are enrolled and regularly attending classes at a school, you may be exempt from social security and Medicare taxes on pay for services performed for that school.   

4.  You are employed due to severe economic hardship, and were issued Form I-688B or Form I-766 by the IRS.

However, if you hold an F, J, M or Q visa but are considered a resident alien (i.e., you’ve been on an F visa for more than 5 calendar years or a J visa for more than 2 calendar years), your pay is subject to social security and Medicare taxes.