Question
We suddenly noticed that one of the oak trees out front (western Oklahoma) has tiny green, yellow, red balls stuck to the underside of most of the leaves.  We have lived here for several years and don't remember seeing this  before.  A second tree has some affected areas, but not as extensive.
We can remove the clumps of balls easily by rubbing the leaf.

Answer
Sounds like insect gall formed by a wasp type insect. The insect lays its eggs on the leaf and secretes a chemical that makes the leaf swell over the eggs forming the gall. These will not harm the overall health of the tree. They may cause the leaves to drop prematurely but this late in the year it will cause no problem. I would not worry about these. They can be controlled with an insecticide but the timing of the spray has to be when the insect lays its eggs and before the gall forms--this would mean knowing exactly which gall forming insect you have. Treatment is not really needed since there is very little harm done to the tree except for the galls on the leaves.