I came to U.S as a student in August 2006 and I was holding a F1 visa. In May 2010, I started working on OPT until October 2011 and since that date I hold a H1B visa. Am I a resident or nonresident for tax purposes?

Your five-year exemption under the F-1 visa expired on 31 December 2010.

On 1 January, 2011, you became liable for the Substantial Presence Test.

Assuming you spent all of 2011 in the United States, you are a resident alien for 2011 and will file your tax return just like a U.S. citizen.