Oak tree roots extend out about three times the height of the tree (note: this is much further than the dripline of the tree.) Thus, a 20 foot tall tree can have roots extending 60 feet out from the trunk. When trees are blown over in a storm, the root system seems small because most of the roots broke off and remain in the soil.
If a live oak root touches the root of another live oak, the roots will readily graft with each other. Red oak roots are less likely to graft together.
Oak wilt spreads readily via these grafted and common root systems at an average rate 75 feet/year. Oak wilt centers have been documented where the disease hardly moves or moves very slowly. Conversely, oak wilt has been documented moving at up to 150 feet/year. There are many variables, such as weather conditions.