Oaks need little trimming. Most homeowners only raise clippers toward an oak tree in order to prune out dead or broken branches. Like other deciduous trees, oaks suffer least if pruned while dormant, but the threat of deadly oak wilt raises special timing issues.
The vascular disease oak wilt attacks all species of oak trees. Beetles spread the virus by carrying spores from one infected tree to open wounds on nearby oaks. The insects are attracted to the fresh sap of an oak's wound.
The sap beetles that spread oak wilt are active from February though May. If you prune an oak during that time, you expose the tree to the risk of catching oak wilt; avoid any oak trimming during those months.
Trim your oak trees earlier in the winter -- February or March -- before the beetles carrying the oak virus are active. As a precaution, cover all prune wounds with tree paint immediately after trimming.