Quick answer
For internationally trained optometrists, Grade B (350) in each of the four OET sub-tests is accepted for GOC (UK) and AHPRA (Australia) registration. AHPRA's minimum for Writing is Grade C+ (300); Listening, Reading and Speaking stay at Grade B. 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.