DA only applies to precision approaches. Precision approaches nearly always take you below the aerodrome OCA/H long before you get to the DA eg CAT 3 ILS etc.. .. .The DA is just a that - a decision altitude. The design of the precision approach allows for the dip below DA after the missed approach is commenced.. .. .In some ILS categories/aircraft it's even possible for the wheels to touch the runway during the missed approach. Obviously an OCA/H is nonsensical here.. .. .OCA/H applies during non-precision approaches. Different categories of a/c have different minimum clearance heights eg Cat A=300', Cat C=400' above the highest obstacle within the circling area (which in turn varies with a/c category eg 1.68nm for Cat A, 2.66nm for Cat B etc). .. .It is with non-precision approaches that the concept of a 'hard floor' to the minimum descent altitude is applied. The MDA is just that - the lowest altitude to which you may descend during the non-precision approach & missed approach. Unless certain defined criteria are met to enable further descent you are just not allowed to go any lower.