I think the most important thing to remember is that you are not a statistic. Each cancer, like each individual is unique.
Oncotype DX was initially developed for women with early-stage invasive breast cancer with ER+ cancers whose lymph nodes do not contain tumor (node-negative). Typically in these cases, treatment with anti-hormonal therapy, such as tamoxifen or aromatase inhibitors, is planned, and the test can help define whether chemotherapy should or should not be added to that anti-hormone treatment.
The problem is that those statistics can only be based on people who have not had a recurrence since they started measuring, which was only about 5 years ago. I have no idea how that 10% statistic was derived, but the more important thing to know is that it only applies to any results within a period of 5 years. The unfortunate, but obviously true thing is that all statistics are like betting on horse a craps table or buying a lottery ticket. Regardless of the odds, the only way you will know if you win or not is by placing the bet.
My recommendation is that you go ahead with the assay (it can't hurt) and then make your decision on chemo based on the risks of having chemo vs. the risks of cancer recurrence, especially 5 years out or longer.