You are completely safe for one year from the date of graduation if you have OPT in hand. Look at the possibility of getting job. If you can work for one year after graduation, then you should try to get into a Ph.D. program in Spring 2008. Or you can go to school early in Fall 2007 (you forfeit the benefits of OPT after you enroll in school. I think just getting I-20 in May to start your course in August doesn't stop you from working until the end of July. If your mentor is willing to extend your assistantship (I assume you have one being in graduate program) along with your I-20 extension, then this will be a good option for you. You can continue working on your thesis and submit a couple of papers to journals for publication. That will make you more competitive as a Ph.D. applicant. However, if your mentor is extending your I-20 just for the shake of keeping you in status and you will have to finance yourself, better apply for OPT right now. Time is running out for you (OPT approval generally takes 3 months).