If i start my OPT in May 2011 and it will end May 2012....If i file my H1 immediately after i get a job, Let's sary around August 2011, and if approved, does it start immediately ? or it starts from October 1, 2011 ? If it starts from only October 1, 2011, can i continue working as F1 OPT until October 1,2011 and from then continue on H1B ? or i've to wait until October 1,2011 to work after my H1B application has been approved....But this doesn't sound right to me cus i won't be able to work during that time.

or how does it work ? i am really lost here. I hope somebody will clear this confusion for me.

Thank you.

 yes u can work. U don't have to wait till October.

 when you file for work authorization you have to mention when you want to start your H1B. There are a few restrictions.. read this:
www.uscis.gov/portal/site/uscis/menuitem.eb1d4c2a3e5b9ac89243c6a7543f6d1a/