I don't understand your assertion that you would eat into the OCH on a CDA.
If you use the OCH as a DH rather than an MDH, then you will dip below it while making the decision to go around and changing the attitude of the aircraft -- typically 50 ft is considered normal. The OCH may be just 246 ft above obstacles or terrain, so by going below it by 50 ft you're eroding that minimum obstacle clearance (I previously used the word "margin", which in PANS-OPS-speak has a different meaning. I meant MOC.)