I don't believe you'll be able to get OSAP in order to pay off old OSAP debt. I'd definitely schedule an appointment to chat with someone at the Office of Student Financial Aid & Scholarships (
http://sfas.mcmaster.ca/) they'll at least tell you your best options. You might be able to spread payments out over a payment plan of some sort to make it more manageable.
Ultimately taking out loans to pay off old debt will put you in a cycle of always owing more because of the interest charged. It's the classic case of 'using peter to pay paul'.
I don't know your exact situation, but speaking generally for anyone that is in a high debt situation and in school, with OSAP not an option, I would suggest talking with family members about the possibility of loaning money to pay off the debt and then paying them back over time (can save paying interest like you would on a bank loan). Any way to avoid taking a loan with high interest, or using credit cards.
There are many part-time job opportunities available on campus. University Technology Services have positions that open each year to monitor the computer labs on campus, and the job is very flexible with work hours to work around your class schedule. The libraries also hire for assistants in the technology areas, and there are job postings up all the time for MSU positions such as working at Union Market, AVTEK, Campus Events, etc. If you're an upper-year student there is also the option of being a T.A. (teaching assistant) for a semester, which is quite a high paying part-time job worth exploring.