Optional practical training

That is something US graduates without citizenship/PR frequently do. They will start their internship on OPT and get the H1b once they have A. step3 B.the paperwork has gone through with the goverment buerocrats. Often, they can't do the entire internship on OPT as frequently the OPT period starts with graduation and they come up a month or two short. One key advantage of this approach is that you save 'H1b time'. Having some extra 'H-time' over at the end of residency/fellowship can be quite helpful.

Btw. your residency should be at a academic teaching hospital. that way your H1b is exempt from the cap and you don't have to worry about that aspect.

 

Yes you need to complete your first year on OPT. Actually have you applied for your EAD card yet? As soon as you get that, they can start the H-1b process.
One thing, some programs I spoke with requires Step 3 before they sponsor H1b, even for US grads, and some programs don't. If Step 3 is required, I would take it early on during internship.