Like any OPT, you might have some grace period (?).

If you can not obtain your H1B approval during that period, you will have to leave this country. But this is true for almost any OPT without pending 485.

So the most important thing for you is to make sure that you get H1B before OPT expiration. It shouldn't be a problem if you have a US Master or higher degree. The worst scenario is to use premium H1B processing, which is only a matter of ~$1000.