You will be filing a non resident return. Form 1040NR.
Since you are still officially a student, when file you will include a copy of your I-20.
In my work, I ALWAYS run into this problem. As a student, even though you are in OPT status, your employer should not be taking out Social Security and Medicare taxes, as long as you have not lived in the U.S. for more than 5 years.
I recommend that you check your pay vouchers and determine if the taxes were taken out. If they were, you can get a refund by completing the attached form, and submitting it along with your 1040NR.
Some employers erroneously believe, that when you apply for an H-1B visa while on OPT, that they can start taking SS and MC taxes. But this is not the case. Your OPT does not end, and your student status does not end, unit you have an I-94 in your passport indicating that you are entered into the U.S. in status H-1B.