Certified GKH Gardening Expert
It will be best to let them finish their entire first year in container, if possible. Bring them inside a garage or somewhere cool, but not frozen.
The next season you can plant them out anytime that the weather is mild, and will be for several weeks so that the tree can establish before harsh weather.
If you leave the tree in a container over winter, make sure it is large enough to accommodate growing tree roots. Roots continue to grow until the ground freezes - well past leaf drop. Roots that circle a container due to lack of space are a serious threat to the tree's health as it grows. Trees are subject to animal browsing especially in winter. Mice gnaw at the slender bark while rabbits and deer chew on branches. Make sure to enclose the tree in hardware cloth that is anchored to the ground. Don't prune it for several years as the seedling needs all its leaf surface to push new growth.
Oaks are susceptible to oak wilt disease. To minimize risk, only prune in winter unless there are damaged limbs which should be pruned off right away.