I just thought i'd give a brief example of what my OH assessment was like. Before i went for mine i'd never had one, and it may help people in a similar position to get some insight...
I had no idea what to expect, but it didn't last very long. I was asked lots of questions and i answered them all as honestly as i could. I didn't lie at all as those sorts of things have a habit of coming back to haunt you and i have no intention of losing my uni place because i lied. The more honest i am the more support i'm likely to get from the beginning because they will be aware of my circumstances (if i am accepted). I mentioned me being admitted to hospital recently, as i got the impression that i could have quite happily sat there, not said anything and walked away but ethically/morally i felt that would have been wrong. He has to get a report from my GP, but the feedback wasn't completly bad. At least i was true to myself.
They didn't take any bloods or check my BP or weight which i expected. Mostly the questions focused on my mental health, the medication i was currently on and how i thought i would cope with the stress and strain of studying something like medicine. The doctor also talking to me about what my offer grades were and things like that which surprised me, but i guess he just wanted a general overview.