A thin coat of pruning seal coats wounds so spores can't stick to the sap and germinate. Seal all oak wounds immediately with pruning 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. A device called Reach-n-Spray by Spectracide allows you to spray cuts made with a 14-foot-pole pruner. This is now available at Home Depot and other retailers for about $20.