The early O'DAY 25 (1975-6) had less ballast than the later ones and so sits higher in the water, around late 1976-7 O'DAY added about 300# more ballast to the K/CB 25 which resulted in the boat sitting about 3" lower (though, strangely, the posted draft never changed?). I knew someone with a 1975 model that despite using a 20" shaft (long) outboard, mentioned that the outboard would ventilate (come partway out of water) in waves. He later had a 1978 model with the OMC inboard and liked the advantage of the deep prop. So, anyway, he advised other 25 owners to use a 25" shaft (X-Long) once they became readily available.
Oh, and your new O'DAY 25 cannot be a 1973 model, first model year was 1975 (Production started in late 1974 though, around August).
These same recommendations about outboard shaft lengths would also apply to the fin-keel model of the 25.