Question
Our Oak Tree has these bright Red balls, on the tops of the leaves. They are only on the sunniest side of the tree. About the size of a cheery tomatoes. None of the other  oaks are infested. Could this be gall?

Answer
Sounds like a gall insect called oak apple gall.
This is one of many leaf galls that affect oaks. These galls usually damage the tree less than do twig galls. However, heavy infestations of this and other leaf galls can cause premature leaf fall and are unsightly on ornamental trees.

Adults are very small and dark cynipid wasps with an oval, compressed abdomen. The larvae are small and globe-shaped.

Galls are about .5 to 2 inches (12 to 50 mm) in diameter, and are filled with a fibrous mass. Each contains a single larva inside a hard center capsule. The galls are produced on the midrib or stem of leaves. Galls formed during spring are green, but become light brown on drying with a thin, papery shell. Oak-apple galls occur principally on red, black, and scarlet oaks.

Oak-apple galls, usually initiated during spring when the young leaf is being formed, sometimes appropriate the entire leaf for its own purpose. The biology is poorly known, but it probably has alternate generations on different host parts.

Natural enemies are usually sufficient to control wasp populations. Galls can be picked or pruned off small ornamental trees.

They generally cause very little health problems with the oak tree. The only thing they could cause is a leaf fall sooner than normal. Really nothing to worry about.