I've got a few bunkers made of Painted 1/2" CDX that have lasted 2 years now. No real visable change in them due to weather.
OSB will shred over time as paint hits it. Used to frequent a field in Anchorage that used OSB on it's target range. We had cut a rather large hole right through the wood within a few months of playing there.
The biggest thing that will help you though is pre-planning, and designing your field on paper. The better prepared you are before you begin the actual building, the smoother it will run. This way you can actually look at different things such as flowing from one field into another (for sectioned off small fields you'd like to combine into one large field on the occassion) as well as conceptual fields to see if they establish the kind of environment you're trying to mimic.
(Also, inflatables work well for their intended purpose. Yes the initial set up can be a pain, but if you do leave them up over a summer, they can be a very good way to have an infinitely modifiable field. Just deflate them at the end of the day, and come out an hour early to inflate them prior to opening.)