Question
I have several oaks, Sand live oak  (Quercus geminata) perhaps. I have
noticed a lot of pure white, flossy material on the cupped underside of the
leaves and in small spaces where twigs leave the trunk. Some leaves are full
to overflowing and the fluff moves easily in the breeze.  I did not see any
insects when gently removing bit of this material, which is slightly sticky
when rubbed between fingers where is disappears. One nearby tree has the
same affliction, but several others do not, though they have small, closed
galls on the tops of the leaves. Can you tell me the cause of these and the
preferred treatment, if necessary. I asked this question earlier but my expert
did not know, then after sending requested photos I never heard back.

Answer
I think it is aphids. These insects have sucking mouth parts and can secrete a waxy material that is white. One of these is the Wooly oak aphid. You should be able to feel the sticky honeydew on the foliage. And it maybe raining down under the tree.
The fluff is waste materila from the aphids.
I would spray the foliage with one of the following insecticides.
Numerous contact insecticides are registered for aphid control. If you do not obtain reasonable control, consider rotation to another insecticide. Contact insecticides currently registered for aphid control include: acephate (Orthene), bifenthrin (Talstar), chlorpyrifos (Dursban),  diazinon, malathion, nicotine sulfate, pyrethrum, rotenone, resmethrin, and tetramethrin + sumithrin.

Or use the soil applied insecticide.
These aphids will not kill the tree but may cause the tree to drop leaves. Aphids have a short life cycle and can reproduce fast so I would spray every 7-10 days for several times. This will kill the newly hatching eggs. This should stop the population.