people my OPT is expiring at the end of October and USCIS advises on its website to leave the country or start attending a school within 60 days. however, I am starting my graduate study from January. if I get my I-20 within 60 days and start my study from January, will it hurt my status? all your input is highly appreciated. thanks a lot.

 You'll be fine as long as the gap between your OPT end date (as per your OPT endorsed I-20 or your EAD card) and your gradschool program start date (stated on your new I-20 you received/wil receive from your gradschool) is less than 60 days. 
Congratulations on making it to gradschool! Which field are you in, if you don't mind?

thank you mitra, it's biochem.