Quick answer
Internationally trained nurses need Grade B (350) in each of the four OET sub-tests for NMC (UK), NMBI (Ireland), and AHPRA (Australia) registration; Grade B in all four is accepted. The NMC and AHPRA accept Grade C+ (300) in Writing with Grade B in Listening, Reading and Speaking. Writing is the most retaken sub-test — typically because of Conciseness & Clarity losses from over-including case notes.