@Payne, I don't understand all the ins and outs, but I think it it's a test that gives an indication of the likely benefit you'd get from chemo. Some of us know we'll need chemo, whether based on tumour size, type, ki67, lymph node involvement, or a combination of all. When it's a little more equivocal, oncodx, or similar tests can be used as a decision aid.
@Milly21, I didn't have it done, as I was node positive at diagnosis, so started with chemo first. Over the years my observation would be that the people who gained the most fro having the test were those whose pathology wasn't clear cut on needing or not needing chemo, and the test gave a clear answer. Unfortunately, like many things, there's no guarantee you'll get that clear answer.
Takec care
Lyn