Quick answer
Internationally trained optometrists need Grade B (350) in each of the four OET sub-tests for GOC (UK) and AHPRA (Australia) registration. The Writing sub-test tests your ability to communicate optometric findings to a non-optometrist addressee — usually a GP or ophthalmologist — in 250 words of structured prose. Most marks are lost on Genre & Style: compressed optometric abbreviations without context.