I believe that once you start med school as an OOS you will always be treated as one.
I asked this question during interview day and this is correct, although they said you could take a year off either before starting medical school or between years 2 and 3 to do a research fellowship during which you are in the state of Iowa for purposes other than education, and thus you can establish residency, although the staff advised against this because you're delaying your medical career and in retrospect saving $20k doesn't mean much.