Hi Livia,
I used the O'Sullivan guide for my preparation as well (I wrote the written exam on September 8th) and I found that O'Sullivan had a large emphasis on numerous topics there were NOT examined on the Canadian practical test. Some of these topics include: wound care, PNF exercises prescription for stroke patients, prosthetics and amputee questions.
From my experience the questions in the Canadian exam were definitely more difficult, required higher order thinking, and more in depth analysis than the O'Sullivan questions. If you are having difficulty with the O'sullivan Practice exams I would suggest you look at the blue print for the Canadian Exam (found on the Alliance website). For whatever reason there was a large emphasis on few but particular conditions. I would review these conditions on the blueprint in depth for your preparation.
Sorry to be the bearer of bad news but I wish someone would have extending this info to me prior to my exam preparation. All the best with your exam prep!
Good luck,
Rory