Knchuu, This is what I got from my International Advisor. YOU have 2 months i.e. 60 days from the day ur OPT expired to get a new I-20 and change ur status to F-1. However, there is no need to go to college for another 5 months. this periods are grace giving by USCIS. Now in your case, u can either just issue a new I -20 and just wiat till October to change ur status back to H1-B. I don't think you have enough time to file for extension of your OPT, since u mentioned ur OPT expires on may 11 which is next week. Your H1-B doesn't start till 1st of Oct. Pratically, u cannot work, but i have seen few folks working during this gap period.

Also, i would recommend you to talk to ur advisor, becoz of so many new rules from USCIS.

Thanks