It is impossible to specialize if you go to an osteopathic school. In fact, if residency programs find out that you even applied to osteopathic school, or even thought about applying to osteopathic school, you will be banished to family practice, probably in rural Alaska, or the Soviet Union.
However, if you attend an MD school, you will be provided with your choice of derm, rads, anesthesia, or ophtho residencies upon matriculation.
Residency match from an allopathic school is more like major college sports recruiting. These derm programs will fly you out to their facility, on their dime, and try and sell you on why you should rank them #1. The program director is allowed one in-home visit with each applicant as well. Occasionally, if you are very highly-regarded (i.e., you passed step 1 on your first or second try, you have a minimal amount of remediations, and you have at least one functioning eye), alumni of these derm programs will buy you a car or a house if you promise to go there.
If you are a poor applicant from an allopathic school, you may have to settle for ortho or neurosurgery.