Hi
I was a student on F1 and joined my company on March 3, 2008 on OPT. My H1 started from Oct 1st 2008. What forms should I file and what category do I come into.

Dheeraj

On F1/OPT you are exempt from residency for 5-years. If you are not married, and you did not complete SPT on H1-B, you must file non-resident tax return Form 1040NR or 1040NR-EZ and Form 8843.

Your F1/OPT income is not subject to FICA taxes, but H1-B income is subject to SS tax and Medicare tax. If you have OPT income you will get deduction based on tax treaty between the U.S. and your country.
Read: Your U.S. Tax Return: U.S. Tax Filing Requirements for Non-Residents