MDA/H is applied to the "Instrument Approach"
OCA/H is applied to the "Visual Manoeuvring"
As I understand it.....If you are flying an approach that requires a "Circle to Land" you fly down to MDA/H.....if visual and in the "Circling Area" you can desend to "Circling Minima" or in some cases OCA/H - which is the lowest minima for VISUAL manoeuver
