It varies with the species of oak. But generally I have to acidify a a bit to just to keep my soil neutral. As the pH becomes more alkaline you run into problems with iron and trace minerals getting bound up chemically, so generally speaking, a slightly acid soil is better.
Here is a nice chart:
http://www.extension.umn.edu/distribution/horticulture/components/1731-29.html
Slightly acid to neutral
Q. palustris - Pin Oak
Q. rubra - Red Oak
Q. velutina - Black Oak
Neutral to slightly alkaline:
Q. alba - White Oak
Q. bicolor - Swamp White Oak
Q. ellipsoidalis - Northern Pin Oak
Q. macrocarpa - Bur Oak
The following chart is numeric and suggests a more acid soil:
http://msucares.com/pubs/is372.htm
A pH target of 6.0 would be good for most of these.
Neither chart lists Quercus Agrifolia. I did find a separate
page on that that listed a wide pH range of 4.0 to 8.0, which ranges from very acid to slightly alkaline. Things are always a bit different in containers, though.
Regards,
Matt.