To the south of British Columbia, the Garry oak is also called the Oregon white oak, and botantists call it Quercus garryana. This oak grows in the rain shadow of the coast ranges and interior valleys from northern California to British Columbia, and in a limited area around the east end of the Columbia Gorge, around The Dalles. It is associated with madrona and Douglas fir in the northern part of its range, and those trees and other oaks (California black oak, tan oak, live oak) in the southern part. As it happens, poison oak also favors warm, dry climates, and its vines climb up white oaks, but as you know, poison oak is not an oak.
The Willamette Valley was largely an expanse of white oak savanna and grasslands before it was settled by Europeans, I have read. Natural and artificial fires discouraged brush and small trees, encouraging the growth of large, scattered trees; white oaks, as well as Douglas fir and ponderosa pine, can withstand repeated moderate fires. In the 150 or so years of fire suppression, many small trees (oaks, firs, and others) have grown up in the old savannas. These trees are relatively tall, with narrow crowns, instead of growing outward with a full crown. In some places one can see a mix of a few old trees with wide crowns and many younger trees; one such place is Bush Park in Salem, Oregon, which must be the center of the white oak world if there is one.