Breast cancer - Can anyone tell me anything about the Oncotype DX test?

Hi all I wonder if there is some one on here who knows a bit more information on oncotype DX 

I had a mastectomy on the 8th of November all went well I then had an appointment today with my consultant who informed me that my biopsy that they had take from my breast was sent to America.

And that when they test it will indicate whether I need chemo or radiotherapy or none at all

Also I have to go back to have a lymph node removed even tho I had two removed when I had my breast removed 3 week ago how come that was missed feel like I'm moving forward