What are these weird growths in my oak trees?

Question

http://www.flickr.com/photos/28739677@N05/2826317044/

I can't figure out what these are? Are they harmful to the tree?

Answer 1

http://www.ca.uky.edu/entomology/entfact...

http://www.ento.psu.edu/extension/factsh...

http://edis.ifas.ufl.edu/IN022

if there's not a whole LOAD of them, they're not going to do much damage to the tree.... just cut them off if they really bug ya....

Answer 2

.They are insect "galls".  Usually on oaks they are caused by gall wasps.  All oak trees have them to some extent.  See the links below for some more info.  

http://edis.ifas.ufl.edu/IN022

http://woodypest.ifas.ufl.edu/galls.htm