I wouldn't use OSB in a chicken coop.
Insulated Concrete............ Better in every way.
http://ichhp.com/
Cheapest initial expense doesn't necessarily mean cheapest long term expense.
I would choose pier and beam over the slab. Ground shifts and settles too much. Easier to occasionally level beam. Plus you have the added benefit of water and sewer line access. If you decide you want to move, on your property, elsewhere, or the direction the front door faces; latch on and move it.
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