If, after OM, the reported RVR falls below minima you can continue to DH for a 'look see'. If there is no DH it means you can land with no decision to be made.
Apparently there are even weirder interpretations of that rule. The local CAA where i work apparently believes if there is no DH you cannot continue to the DH since it doesn't exist and therefore anytime the RVR drops below 75m after passing the OM or an equivalent fix, a go around is mandatory. For france we used a blanket 20ft DH for airbii and 50ft for Boeings. But apparently the french have changed their way and we can now use the normal no DH.