I don't know what it stands for AON off the top of my head but they're a series of game-like tests that are meant to assess certain skills. For instance in one you're tested for situational awareness my managing three simple tasks within a time limit. I know AA and Delta use them and I think UA does as well.

As for the competitive mins, I don't know if anyone can really answer that. It seems like to me some sort of PIC turbine time helps, but beyond that everyone I've seen who've received a CJO had backgrounds and experiences all over the place. All I can say is if you have the minimum time requirements and you're interested, throw your app in and see what happens. People have been hired from all over the spectrum.