off campus work permit is given to you during your study. It allows to work 20 hours a week. Whereas, Post graduate work permit is given after you complete your study. It allows you to work full time ( 40 hours or more).
OCWP allows you to work while study. You can work a maximum of 20 hours per week during regular semester and full time during breaks. It is valid till the time your Study permit is valid.
PGWP is granted after you complete your studies. You have to apply for it within 90 days of your course completion. Its validity depends on the length of your course. You are entitled to get PGWP of 3 years if your course is 2 years or more and your PGWP will equal to the length of the course if the course is less than 2 years and more than 8 months.
Both OCWP and PGWP are issued without any requirement of job at hand.
you have to have 6 months in school before you can apply for an off campus work permit. also you have to maintain a GPA of 3 in order to apply. hope that information help.