A thin coat of pruning seal coats pruning cuts and wounds so the fungal spores can't stick to the sap, grow into the wound and infect the oak. Seal all oak wounds immediately with pruning seal or black paint regardless of the season. Most wounds occur through pruning, but lawn mower scrapes and broken branches should also be sealed as well as the tops of freshly cut stumps.
When pruning, spray each wound right after you make the cut so none are missed. There is no need to seal old wounds because they dry out within a few days. Pruning seal is not recommended for trees other than oaks.
You can buy pruning seal at most hardware stores that carry garden supplies. When using a long pole pruner, you also need a device with a long reach to apply paint to the cuts. You can build a homemade spray can adapter following these instructions or search online for "spray can pole adapter" or "long reach universal sprayer."
Sadly, the Reach-n-Spray by Spectracide which was sold at the Bee Caves Home Depot is no longer available online or in-store as of November 2020.