Well known in the Eugene area on Oregon white oak (Garry oak). Looks like it's caused by the oak gall wasp Besbicus mirabilis. Glad to finally know what's behind these "pop balls"!
I would say Oak Galls... they are caused by gall wasp larvae. Here is a link of quite a few different types of Oak Galls...
http://joycegross.com/galls_ca_oak.php
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