you can try doing this. take u'r I797 C notice of action slip to the international student advisor office and ask him to write a letter explaining u'r OPT is already active and valid but u havent received it yet its on its way. If that dont work u'r advisor might provide u other suggestion like contacting USCIS (or whoever) to fax a temporaryucopy of work permit to show it to u'r employer.