Herluf. .. .The OCH is the height of any obstacles plus an appropriate margin or minimum obstacle clearance. It doesn't take into account any limitations of the navigational aids used.. .. .The decision height that you are required to use (at least in the UK, in the absence of company issued minima) is the higher of the OCH and the system minimum. For a Cat 1 ILS presision approach the system minimum is 200 ft. . .. .Similarly the MDH for a non-precision approach is the higher of the OCH and the system minimum, which is 250 ft for a localizer approach, 300 ft for an NDB or VOR approach etc.. .. .For the UK, system minima are published in the AIP AD 1.1.2.. .. ....which begs the question "why put OCHs lower than the system minimum on the approach chart?" I suppose it could be argued that the information that the DH is limited by OCH rather than system minimum is useful to the pilot, but I suspect it's more of a historical anomaly.