I have been a student on F1 visa for 5 years. So now legally I can file taxes as a resident alien. Now I am working on my OPT. I am not sure if I have to pay social security and medicare. According to IRS I am exempt till I am a resident alien and as long as I am on F1 I am exempt from the substantial presence. However, since I file taxes as resident alien does that legally bind me to social security? I am totally confused... help

Thanks in advance

Because you are now consideded a resident alien (irrespective of visa status), you ARE liable for Social Security and Medicare taxes.

You have the legal obligation to inform your OPT employer of this fact when you begin work so that he knows to withhold these taxes from your pay. Recommend that you do so in writing so that you have a record of it.