If you are 100% set on Radiology or any extremely competitive specialty, you need to go to MD school. I can tell Program Director after Program Director that we learn the same things in DO school as MD school, but they just don't believe me and to them, DO's are an unknown. And with radiology being so competitive, "Why take that chance?" I believe is their opinion. One PD in particular asked me, "What do you guys learn in DO school?" while I was on a visiting rotation. I told the PD my curriculum, but they flat out told me at the program that they really didn't take DO's. I didn't even get an interview there.
There are some programs that are open to taking a DO here and there I guess so like the above posters said, "It IS possible to match allo rads as a DO." But you really have to work harder. It's very frustrating to work so hard and be thought of as a lesser applicant, only because of the "D.O."
That being said, I'm glad I went to DO school, because it was a great experience. Had I enjoyed clinical medicine more (which I really thought I was going to) I would be even more grateful as a DO, because some of the OMM techniques can be very helpful. But I realized I liked radiology much much more and was fortunate enough to match into my #1 allo rads spot.
For those that are already in DO school, if you are really interested in radiology, and considering the DO rads programs, you MUST rotate through their departments to even be considered. If you are going to apply to allo rads, do your best on Step 1 and cross your fingers.