If you’re lucky enough to have a native California Oak, you should be aware that it can suffer from both over- and under-watering.
California Oaks get all the water they need from winter rains. During winter drought periods, however, they’ll need deep watering. Employ a soaker hose or drip irrigation to release water slowly over a several hours to get the water down deep into the soil. Connect the soaker hose to a hose bibb, or better yet, to rain barrels or cisterns used to collect storm water.
https://latimesblogs.latimes.com/home_blog/2011/08/soaker-hoses.html
This deep watering during dry winters is important because drought-stricken oaks are more susceptible to pests, such as the shot hole borer, and other diseases.
https://ucanr.edu/sites/pshb/
Once summer has arrived, though, you’ll need to cut back or even stop watering, especially if your oak is mature. Too much summer water promotes oak root rot fungus that causes Sudden Oak Death disease.
http://www.suddenoakdeath.org/
Because of the sensitivity of California Oaks to excessive summer watering, it’s crucial to select plants that can tolerate summer dry conditions when planting a garden underneath your tree.
California native plants make perfect companions for oak underplanting. Like the oaks, they require water only during the winter months when we get most of our rainfall. Once established, they can survive the summer with no supplemental irrigation.
One common mistake people make with California oaks is to remove the fallen leaves. Don’t do this, as the oak leaf mulch provides extraordinary benefits: the litter decomposes and feeds the soil organisms which in turn feed the tree, while also protecting the shallow surface roots of the oak. Allow the oak leaf mulch to accumulate under the tree to a depth of at least four inches. Just clear it away from the bases of the native plants you’ve chosen. For more information on oak care:
http://californiaoaks.org/
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