But the use of a CDA is predicated on MDA+50 as the minina, is has to be. Where did OCH=DH come into scope?
No, it's not, not with the "New" EU-OPS scheme. See post #10 above.
The EUOPS reference that 172 refers to is the latest legislation. Note there is now no requirement to add 50 ft for any reason.
The Old Appendix split NPAs with an MDH from precision approaches with a DH. The New Appendix does not:
Appendix 1 (New) to OPS 1.430
(b) Category I, APV and non-precision approach operations
...
2. A non-precision approach (NPA) operation is an instrument approach using any of the facilities described in Table 3 (System minima), with a MDH or DH not lower than 250 ft and an RVR/CMV of not less than 750 m, unless accepted by the Authority.
...
4. Decision height (DH). An operator must ensure that the decision height to be used for an approach is not lower than:
(i) the minimum height to which the approach aid can be used without the required visual reference; or
(ii) the OCH for the category of aeroplane; or
(iii) the published approach procedure decision height where applicable; or
(iv) 200 ft for Category I approach operations; or
(v) the system minimum in Table 3; or
(vi) the lowest decision height specified in the Aeroplane Flight Manual (AFM) or equivalent document, if stated; whichever is higher.
While EU-OPS 1.430(d)2 is quite emphatic about the need to fly NPAs as CDFA, it doesn't do a good job of explaining that such CDFA NPAs use a DH, not an MDH. But that is the way it is expected to be.