re: What is this on my live oak trees?

Leafy oak gall made by the leafy oak gall wasp. It won't harm the tree, just the leaf it is on to some extent.

Being that they are young trees, if they show up in large numbers you can just pinch them off of the leaf, trying to save be leaf, they're attached to. There's probably another, even smaller parasitic wasp that targets those specific galls. Oaks are cool because they attract everything, and that's the point.