Does anyone in here know how long can one stay in the US after OPT expiration (legally)? 
 I have heard people saying 60 and/or 90 days?? Anyone with the CORRECT information?

Thank you,

 You have 60 days to transfer to different college or prepare for departure. 60 days. 

Thank you for your response to my earlier question.

Do you have to start attending school within those 60 days?

What if I already have an I20 from one school but  waiting on a response from another school? Any ideas?

 if you already have an I-20 from a school, you're good. that 60 day deadline doesn't apply for you.

 If you have an I-20 from a college, you've already been officially accepted to that college and your SEVIS records updated, and you will start school when the semester starts, so 60 days doesnt apply here. 60 day rule applies in cases when your OPT has expired and you havent yet enrolled at a college and obtained an I-20.

However, if you're waiting to hear from another school where you have or plan to apply, and if they accept you, you very well 'might' have to go through the process of getting SEVIS release from the school that has currently issued you the I-20 and then only be able to get a new I-20 from the second school - please consult with the second school as to what their policy or process is regarding situations like yours.

Good Luck.

 hey guys
I was so screwed up with this USCIS. My OPT expired on Sept 20. I have been accepted by the University and worked on all documents and submitted by Nov 14. The unviersity was ready to issue me I-20. They were aware my 60 days period. My SEVIS was released from previous university and everything was ready to issue I-20But they couldnt issue me I-20 because I was already out of status for more than 2 weeks. What happened was on my SEVIS record my OPT expired on Aug 31. But the EAD card says my OPT expired on Sept. 20. The DSO told me that she sent an email to SEVIS people to correct the date. Technically I am screwed now. But DSO assured me to be patient and wait or few days before they fix it. Even if they dont fix it we would have to file for reinstatement. So I'm hoping that I dont have to file for reinstatement. That would suck. I tried to do everything right but you know sometimes it goes against you. Anyway hoping for the best.
p.s. does anybody know how much is the filing fee for reinstatement?

So you have to have I20 before OPT expires?? Or can you use that 60 days grace period to obtain one.?

And thank you everyone that has responded. Mangale, if the DSO is in communication with US

The sevis office, then it should be alright. Good luck!