I'm in my 2nd year at UNECOM, and I don't remember there being a D.O. letter requirement - I definitely didn't have one and got in, so that must be new. As far as the "professional" requirement, I called them when I applied and asked about that, and they said it basically means "not a professor". It can be someone who you do research for, a volunteer supervisor, your employer, a doc you shadowed, etc. I sent a letter from the social worker I worked with from Big Brothers Big Sisters.