Question
My pin oak tree up to now has been very healthy. It still looks good, but this summer we noticed growths on the branches that look like poop. What are these growths, the cause, and how do I get rid of them?

Answer
These are gall insects. Galls are irregular plant growths which are stimulated by the reaction between plant hormones and powerful growth regulating chemicals produced by some insects or mites. Galls may occur on leaves, bark, flowers, buds, acorns, or roots. Leaf and twig galls are most noticeable. The inhabitant gains its nutrients from the inner gall tissue. Galls also provide some protection from natural enemies and insecticide sprays. Important details of the life cycles of many gall-makers are not known so specific recommendations to time control measures most effectively are not available.

Generally, insecticidal control is not satisfactory because the wasps are physically protected within the galls. Correctly timing applications to provide effective preventive control is difficult. Where practical, pruning of infested twigs may help to reduce the problem on lightly-infested trees. However, pruning is impractical if large trees are heavily infested.
I would not be too concerned with the galls they really do little to harm the tree. Here is web site that gives more information on oak galls. http://www.ento.psu.edu/extension/factsheets/gallsOnOak.htm