I have never bothered with the Prep Test on Oboe. Quite frankly, my own view is that by the time someone has mastered the Prep Test (unless you simply want the experience of entering the exam room when you know you cannot "fail" - or pass for that matter) they are ready to take Grade 1.
I'd consider Grade 1 and give your pupil a challenge.....

That said, I do use the Prep Test for my pianists a lot - there is seems to have much greater value on the road to Grade 1.