You did not provide any specifics so I will answer your question in general terms. OPT to H is all about timing. After your OPT expires you have 60 days to either begin a new program of study, begin employment under a new visa or leave the country. To the best of my knowledge you may not remain in the country indefinately just because the H cap has been reached.
Staying in the U.S. after your 60 day post OPT period without enroling in another program of study or LEGALLY begining employment can have 2 consequences and both are bad. 1. You risk violating the visa overstay regulation which could affect a subsequent green card application. 2. You risk working illegally because unless you have another source of funding one would have to wonder how you are supporting yourself.
Bottom line: if you do not have a job with an employer who has filed a timely H application on your behalf (or can employ you as a J-1) and are not planning on returning to school after your OPT ends you should consider going home.
what if someone already has a job? The OPT expires on May 15 2007 - Will it help if the employer files for premium processing? I was told that once the approval notice is in hand the preson can continue working although the H1b actually starts from Oct 1 2007. The problem is how to remain legal from May 15 2007(date when OPT gets over) till Oct 1 2007.