I have applied OPT extension to USCIS last month. It is been approximately 35 daysI have received my receipt. I don't know how long it will take to complete, but I am worried what will happen if I didn't receive OPT extension until my current OPT expires? under which circumstances, I will still be legal to stay here?  I read about Cap-Gap, but until the OPT is approved or denied will this be valid for me? 

Today, I called USCIS and a lady has been saying I might need to go to have a USCIS to have finger print...bla bla... None of my friends had done that. Does anyone faced same situation like me??

Thank you,