Probably could use a new roof. I would make it so that you can access inside to clean (if it isn't already set up like that). I like the colors. I have seen duck doors where the hinge is on the bottom and it folds down to make a ramp of sorts.
The one with osb will need more maintenance (really good paint touched up yearly) unless you add siding to it as the osb is not meant to be used directly outside. It will break down sooner.
For what your needs are, either will work.
Have you looked here at the byc duck sticky: https://www.backyardchickens.com/threads/duck-pond-pool-pictures.320654/
Also, I would agree, it sounds like a modified dog house would be best in your situation (adding a door to it if needed).