Do you know what taxes I have to pay if I work under my OPT and I am from Greece?
I know that federal taxes can be reduced based on the special agreement between USA and each country.

Thanks in advance.

There IS a Greece-U.S. Tax Treaty, but a quick check shows no treaty exemptions available for student.

Sorry!

On F1/OPT you are exempt from residency for 5-years. You must file non-resident tax return Form 1040NR or 1040NR-EZ and Form 8843. Your income is not subject to FICA taxes but you must pay federal and state income taxes.

On your OPT income you will get deduction based on tax treaty between the U.S. and your country. And if there is no tax treaty, you do not get the deduction.
Read: Your U.S. Tax Return: U.S. Tax Filing Requirements for Non-Residents