Ore seems to be controlled entirely by exact security status and region.
In reality, there is a many-digit number that's rounded to produce the sec status you see on the map. It ranges from -1 to +1, and anything below 0 is displayed as 0.0. A constellation has a general hidden sec rating, and all the systems in it are generally within +- 0.15 of that number.
Of the two constellations in the author's territory that are below -0.5, where we do most of our work, there is up to ochre in -0.52, crokite being present in -0.56, and mercoxit being present in -0.77 (with quantities of +10% crok). No arkonor or bistot found yet, but no system below -0.77 has been explored by the author.
Edit: This sytem has been revamped and is no longer valid, CCP uses a new ranking system involving letters. I.E. instead of -0.52 the system would be 0.0a 0.0b 0.0c etc.. The below link is now invalid as well.