You have 60 days after expiration of your OPT that you can lawfully remain in the US.  Depending on when that would run out and whether it would take you into the H-1B validity period (assuming you find a H-1B employer and the petition on your behalf is approved) you may either be able to remain in the US until the H-1B becomes active or have to leave to consular process for your H-1B visa.