i think ur getting confused between SIN and OCWP ...SIN is social insurance number . I am guessing u got a mail saying ur OCWP is approved...once u get the work permit in mail in a few days u take it to service canada center closest to u and apply for SIN u get the SIN on the same day if u go to their office. once u have OCWP(off campus work permit) and SIN u start working if u have a job. ur employer will need a copy of work permit, SIN number/copy ,ur passport and visa.
http://www.servicecanada.gc.ca/eng/sc/sin/