Quick answer
For internationally trained radiographers, Grade B (350) in each of the four OET sub-tests is accepted for HCPC (UK) and AHPRA (Australia) registration. AHPRA's minimum for Writing is Grade C+ (300); Listening, Reading and Speaking stay at Grade B. Writing is the most retaken sub-test — typically because radiographers default to structured imaging-report format (Indication / Technique / Findings / Conclusion) when the OET requires flowing paragraph prose. The criterion most often lost is Genre & Style.