To work an OPT job, the school must first recommend to USCIS (immigration officials) that you receive permission to work in the U.S.   The school makes its recommendation on Form I-20 and sends it to immigration with a Form I-765. The Form I-765 is a request for a work authorization document or work permit. If USCIS approves your work permit then you will get a card in the mail and can use it to work off-campus under OPT.  Note: you aren’t eligible for OPT if you have already worked in CPT status.