I'd suggest more ventilation... true, your coop is not going to be cooler than the outdoor daytime air for any length of time, BUT really good airflow (and no offense meant, but yours is a bit minimal) will give you two valuable things: 1) a breeze, which helps b/c a chicken in the shade in a breeze is a lot cooler chicken than one in hot *still* shade; and b) the coop will cool off as quickly as the ambient air does, rather than remaining hot during cloudy hours or after the sun goes down.
Insulating will help some, especially insulating under the roof if it's bare metal there, but really cutting one or two big ol' holes in the walls and covering 'em with mesh may give you as much or more bang for your buck. The cut-out sections can be kept to be bolted back on for winter. I wouldn't do just OSB, though, it doesn't have that much insulating value for the work involved in putting it up.
Good luck, have fun,
Pat