Yes to all points, except maybe the OSB/chip board.
Chickens produce a certain amount of moisture, and moist conditions can sometimes cause the OSB/chip board to come apart, especially if something picks at it. It might turn out to be fine, or it might not, but I would at least think twice...
Yes, that makes sense.
I've had chickens in outdoor pens, with walls of just wire mesh, in situations where foxes came sniffing around regularly, and sometimes tried to dig underneath. It didn't seem to make much difference: the chickens acted stressed if the fox was right there, but were fine...
You can probably build the coop with just hardware cloth and some framing, since it is inside the barn. The barn provides weather protection, the hardware cloth would contain the chickens and provide protection from predators.
Here's an article about someone's coop that is part of a shed. After...