Some really good advice already given. The most important of which is the knowledge of GN3. It really cant be emphasised how much depth of knowledge you will need of this book. If you are taking the Oct course, then you need to start now a couple of hours a day minimum reading this book, get used to all the correct terminology, stop using everyday slang like "megger" "IR test" and so on. You will need an in depth knowledge of every test instrument, its range, purpose and its operating voltage and current etc for each test. Dont dwell on the practical part, you will get plenty of practice prior to the real test, and then you will be coaxed through, unlike the written exam which is a very testing closed book on your own exam. Digest GN3 and you will have no problems.
The Oct 2391 is my debut at delivering the course, i am looking forward to the challenge, but i am also re reading GN3 several times, because as the saying goes "You cant preach what you cant practice".
Cheers..............Howard