That's the question that my lovely YouTube student, A. Miyan asked me, and it's a really important question to ask.
Really, you need to have a B2 + or C1 level of English - in other words, be upper-intermediate.
That's important. If you take the test before you're ready and before you have enough of a grammar and vocabulary bank to support you, it will just lead to frustration and pain.
The OET is not like the IELTS in that you have to "show off" to the examiner and demonstrate your grammatical knowledge and range of lexis by using complex grammar. I always tell my IELTS students to start at least one or two sentences with an "if" for example, because this means that they will use a conditional, and so get a 'tick'. They should try and use passives in their writing and some relative clauses will help enormously.
The OET, however, is not like that. You don't need beautiful long sentences to get a high score. All you need to be is be:
accurate
appropriate
efficient
Being accurate in the OET
For example, talking about an appointment a patient will attend won't help a reader if the appointment actually took place last week. It's important to get tenses right and singular and plurals correct, especially as using the wrong choice may affect meaning.
His kidney is failing has a very different meaning from his kidneys are failing.
Being appropriate in the OET
It's also important to use appropriate language. In your OET speaking sub-test, you wouldn't use the term "analgesics" with a patient, even though it's a great medical word. You wouldn't use it because it is too medical. It wouldn't be appropriate. Instead, you should use "painkillers" as that would be far more easily understood.
Being efficient in the OET
And don't forget, don't use 3 long, complex sentences that are beautifully crafted, when one will do the trick. It's much better to be efficient and get to the point, rather than use extra words which don't really add to the meaning.
So, having a good level of grammar and knowledge and having those upper-intermediate skills will not just help you in showing what you can do, but will give you the confidence to show you what you shouldn't do, and that's just as important when trying to crack the OET!