Both OCA/H and MDA/H are related to instrument approaches - precision, non precision and visual manouvering (despite its name it's IFR procedure). MDA/H is derived from OCA/H by adding the margin, which is based on operational consideration of:
-category of operation
-ground/airborne equipment characteristics
-crew qualifications
-aircraft performance
-meteorological conditions
-aerodrome characteristics
-terrain profile/radio altimeter
-pressure error/pressure altimeter
-location of guidance aid relative to runway
-etc.
Descending to MDA and flying level to MAPt is simply not a way to handle modern transport category airplane. While it's not unlawful to do so, my SOP prohibits leveling off at MDA and go-arond must be started at VDP latest. Anyway, intentional descent to OCA/H is a big no-no if your margin is greater than zero.