Hi Ravendark,
I don't know exactly how it works in Ontario but I can tell you how it works in Alberta, it shouldn't be so different.
First of all you have to have your transcripts evaluated by an authorized agency that will send the report to the national dental assistants examining board (NDAEB), so you can register to take the national dental assistants exam. After you pass that exam, it depends what are the regulations of the province that you live in...they will either let you register as a dental assistant or make you take a clinical exam. Which I think by 2007 will be mandatory. All the detailed information and exam dates can be found in there website
http://www.ndaeb.ca/
The other route you can go is just try to work without being registered but at least here in Alberta is really hard to find a dentist that will hire an unregistered assistant...and you can not perform any intra-oral duties, just chair assist.
It takes around a year till you get all this burocracy out of the way..
To make sure just phone the ontario dental assistants association and ask their regulations.
Hope that helps...any other questions let me know and I'll answer to the best of my abilities...good luck