A cat III approach has no OCH, only obstacle free zones! Hence if an OCH is published no CAT III allowed, same for a CAT II approach with an amended by notam OCH above 100.
Have some background info on this but unfortunately unable to copy/paste.
BOAC is right though
DA(H) are system, state or crew minima and as such published, however if a OCA(H) exists which is higher than those minima then the higher value must be used ( limitations regarding CAT II/III observed as in my first post).
Operations below DA/MDA are see and avoid, hence an OCA! For operation below DA on cat II/III see and avoid is not acceptable.
Hope this makes sense, I find it easier to understand than to explain!