It is imperative that Oak and Elm trees get pruned while in the dormant season. These species are at high risk of disease, namely Oak Wilt and Dutch Elm Disease. Trimming them in the wrong season can greatly increase your risk of these disease as the diseases and the pests that carry the diseases will be active. The fresh pruning cuts in the dormant season will be hardened off and will no longer be easy access points by the time Spring roles around. Every season is a little different, so if you're not sure if you're in a safe window, ask an arborist!