Painting that OSB or chipboard as you called it is very time consuming. It will take about 4 or 5 coats of paint and 3 of those will be Kilz primer and what ever color your finish coats will be.
I suggest water-based primer and paints..less smelly paint fumes. And do at least remove one of the panels to see if any insulation is back there. You might even consider putting up drywall over the chipboard. Some extra work around the doors and windows would be needed if you do it that way.
And what about the garage door? and the tracks? Are you planning on replacing it with a wall and a door?
I think floor to ceiling shelves for your tubs is an excellent idea. Maybe some counter-space for other crafts.