Trees, I don't know. But some large shrubs in my yard that exist under or near oaks that are drought resistant and provide some screen are: cottoneaster, privet, sweet olive.

Toyon (Heteromeles arbutifolia).

My first thought was sugar bush or manzanita. Check out what grows under oaks in your area.....it should probably be a native.

Look on the Las Pilitas website to get ideas. I think Garrya, which is a large growing shrub with tassels on it, native coffeeberries or Ribes (gooseberries).

Snowberries might work too. These are large shrubs, not trees, but are quite large. Whatever you do, you don't want to start watering near the tree. Good luck!

Thank you so much for your ideas. I have some manzanitas in the yard - beautiful, but very slow growing. This site is under 3 dense oak trees, so I am concerned that there is not enough sun for sweet olive or manzanita to grow well. I have a couple of lovely ribes, they also like more sun I think than this site would offer. I need something that might reach 10 to 20 feet..

As for the bay laurel that the nursery already sold me, does anyone know if it CAN grow in poor soil and very little water and not much light? The californa bays can do ok, but my reading says the bay laurel likes rich soil, some water, and does better with sun.

I prune my oaks to allow filtered light, but 2 of these oaks are base in the neighbor's yard, and so they are thick and dense and unpruned.

I have a few coffeberries also, they are languishing in the lack of light...

I will look up privet and cotton easter. Do you think the pittosporums could be transplanted? Would they languish in the shade too?

I have a whole hedge of pittosporum undulatum under my oak trees. They are not leggy but they do require pruning, otherwise they get leggy. They need to be consistently topped so that they don't try to grow above the oaks. That keeps them nice and bushy.

undulatum seems to be quite content to grow in this setting. The oaks are pruned to allow filtered light. I even get very nice fragrant blooms in the Spring.