You must confirm with your employer if the H1-B will be filed as a change-of status or via consular processing. You may be eligible for one of two cap-gap extension benefits:

• If the employer filed your H-1B (change-of-status) petition and it is received by USCIS prior to your post-completion OPT expiration date, you qualify for an extension of your OPT employment authorization.
• If the employer filed your H-1B (change-of-status) petition and it is received by USCIS after your post-completion OPT expires but during your 60 day grace period following OPT, your F-1 status and permission to remain in the US are extended but you are not eligible to work.