Never use oak for any structural or bed wood that will be exposed to the weather. Oak reacts to moisture and will warp.
All wood reacts with moisture and warps.
The dimensional change coefficient of white oak is .00365 as compared to.00265 for southern yellow pine, which means oak will warp ~ 30% more than pine. However white oak is still entirely suitable for outdoor use. In Wisconsin we use it for truck and trailer decks, truck side boards, horse fencing, barn beams, landscaping, and packaging, and it works fine.
