I am an OR nurse in Ontario. As far as I know, it is either you have to have previous experience in the OR or taken the Periop course. Never heard of 'on the job' training around here.
I def recommend to take the Periop course, there is a lot to learn about being in the OR and I found the course very helpful in preparing me for the workplace. I can't even imagine going into the OR and not even knowing what the instruments are called or sterile technique in donning/doffing. The Periop course will give you the fundamental knowledge.