Quick answer
Internationally trained radiographers need Grade B (350) in each of the four OET sub-tests for HCPC (UK) and AHPRA (Australia) registration. No Grade C+ concession applies. 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.