It is always difficult to find plants to grow under oak trees where irrigation cannot be used. The best plan is to stick to native plants. However, following is a short list of appropriate drought tolerant plants that will look good together. Achmaeas and Bromeliads will need a sprinkle into their cups in the summer and fall from time to time, but it does not require damp soil.
Achmaeas and Bromeliads
Dudleya lanceolata (California native)
Gasteria Mahonia repens (California native)
Salvia Spathacea (California native)
Symphoricarpos, any (California native)
Here is a California live oak with a climbing rose, Rosa banksiae lutea, weaving through its branches. It’s wonderful to behold in springtime, and in summer the rose lies dormant and does not impact the oak at all. The rose is quite old and grows with no irrigation at all, its roots shaded by the sheltering oak.
Careful plant choices prove that you can mix native and non-native plants with great success. Patience is important as drought tolerant plants such as this rose must grow slowly because the root system must plunge deep into the ground in order to survive the long dry summers.
This particular rose requires no pruning as all of the growth is up in the tree. As the rose grows slowly, without irrigation, economy of branches is built in.
Below, on the left you will see Salvia spathacea which has a nice lush look for a dry shade garden. Although slow in its first year, this salvia spreads laterally from roots and will make a pretty ground cover in time.
Symphoricarpos mollis can be found in California as well as all over the western states and its form and color varies slightly in all the different microclimates it enjoys. In Malibu it is very green and lush under oaks in winter.