RadOnc is an extremely difficult specialty to match into, even as an MD student.
From the 2011 NRMP numbers, the average USMLE Step 1 score for a US MD student who matched into rad-onc was 240 (independent applicants, which FMGs and DOs will fall under, mean USMLE was 241). Mean USMLE Step 2 score was 242. 22% of US MDs who matched radonc have PhDs (and another 8% have graduate degrees). 45% of matched US MD students came from a Top 40 NIH funded medical school.
The mean number of abstracts, presentations, and publications that a successful US MD student had was 8.3 (unmatched was 6.6), and for independent applicants, it was 9.6.
http://www.nrmp.org/data/chartingoutcomes2011.pdf
Realistically, as a DO student, while not impossible, it is extremely difficult (and uphill battle) to get into a radonc residency. There are fewer mentors who are osteopathic radiation oncologists, and none of the DO schools are large NIH-heavy research institutions (not even *MSU/OSU). If you attend a DO school near a major city with lots of research facilities (Philadelphia, New York, Chicago, Lansing, Southern California), you can do research at those institutions (while attending medical school). If your school is located in a rural location, it may be harder to do research related to radiation-oncology (or even find radiation oncologist mentors, MD or DO)
There are DOs out there who are radonc residents and practicing radiation oncologists so it's possible - but it is an uphill battle.
One of the most respected radiation-oncologist (that was briefly referenced in an above post) is Paul Wallner, DO, FASTRO, a PCOM graduate (1968), and a recipient of ASTRO's Gold Medal, the highest honor that can be bestowed on its members. He is a former Chief of Clinical Radiation Oncology Branch, Radiation Research Program at the National Cancer Institute in Bethesda, MD. He is a former chairman of radiation oncology at Cooper University Hospital/Robert Wood Johnson, and former Vice Chair of Radiation Oncology at the University of Pennsylvania.
*Michigan State University received $49.5 million in NIH funding in fiscal year 2012
*University of Michigan received $468.5 million in NIH funding in fiscal year 2012
*University of Oklahoma received $40.6 million in NIH funding in fiscal year 2012
*Oklahoma State University received $4.5 million in NIH funding in fiscal year 2012