You are aware that it is hazardous to the health of old native California species of Quercus to irrigate for new plantings? Trying to get a tree established within the tree root zone implies the need for more water than it may be accustomed to. That said, it may be useful to consider drought and shade tolerant native large shrubs/wall trees such as Toyon or Pacific Wax Myrtle or perhaps Lemonade Berry for such screening. If you do need to provide supplemental irrigation, keep it confined to a limited drip system well away from the oak's trunk, and minimize wet soil under warm temperatures, which lead to oak root fungus and death/decline of the tree over time. If the oak has already been subjected to years of irrigation within the canopy starting from its youth, and your soils slope and/or are well drained, and the oak is healthy and appears to be tolerating summer irrigation you have many more choices.
Fast growing large shrubs such as Pittosporum spp's or Xylosma or Coprosma depend might be useful, and require relatively little summer irrigation in shaded conditions once established. A Hoheria or Mayten tree might also work for you.

I recommend waiting a month. I was horrified when I removed my palms and suddenly had a view of the wires and a big ugly transformer. I immediately started thinking about ways to disguise it. A few months later I don't even see it. My mind screened it out. You may have the same experience. If it still bugs you in a month, you can still plant something from the list you are now developing, and October is such a good month for planting.

Renee

Bahia - THANK YOU for your suggestions. And yes, I am well aware of the dreaded black fungus rot that occurs when overwatering near the root system of a sacred oak. That's why I LOVE your suggestion of a drought tolerant Toyon. You just made my life easier today!

In California you should assume the fungus Armillaria is in the soil. It is activated with moisture at a time when the soil temperatures are high. The fungus feeds on roots, dead or alive. When the brown mushrooms are noticed at the point where the trunk enters the soil, the roots are damaged beyond saving the tree. After the tree falls over and is made into firewood the fungus continues to live on, feeding on the dead roots still in the ground. We lose black oaks in our forest every year from Armillaria even though there is no summer water, only a drainage problem. Al

My old growth oak is in perfect health. It is truly a majestic tree that has been pruned to provide a light canopy of filtered sun.

Bahia suggested the Toyon berry tree, which I really like the idea of because it can become a lovely sculpted tree eventually.

Unfortunately, my partner checked it out and said it would take too long to grow. Someone recommended a camelia instead, trained to be a tree -- not a shrub. However, I am concerned that a camelia will require too much water next to the oak.

What do you all think?

A manzanita would look amazing. I'm not sure how tall you want this new tree to grow, but there are some very tall-growing manzanitas.

Univ of CA has several pubs available about planting under oaks.

I've seen majestic specimens die within 6 months after they were irrigated for whatever the reason.

Be wise, read & learn before you plant. I doubt you want to talk about your previously majestic, but now failing, oak!

check the pubs here
http://books.google.com/books/p/anr_publications?q=growing%20under%20oaks&hl=en&source=books-referral-partner

Here is a link that might be useful: planting under oaks in CA

It is a fact that Toyon is not available in specimen sizes or particular ularly fast to reach mature size. If you really want something big and fast, you may be better off placing a raised planter of perhaps 4 foot square by 2 foot deep internal dimensions as a raised planter in the location you want a screening tree, which could be spot irrigated and be less likely to be a hazard to the oak. A Mexican Weeping Bamboo might be a perfect choice for such a planter,and a 15 gallon bamboo would already have some good height.

Can you use fencing instead of a plant? A tall, wide open trellis might hide the lines enough and you wouldn't need to worry about fungus. I have a big wood arbor that hides the neighbor's motorhome.
Renee

Thank you all for these excellent suggestions. I've filed these away and will be researching all my options before I plant anything, I can assure you, based on your ideas and recommendations!

I agree with central cali's suggestion for a manzanita - there are so many to choose from. Also, Rhamnus Californica coffeeberry can grow into a very large rounded bush, as well as a Cercis California, or scrub oak. I'm looking at several of these beauties from my window as I type. I definately would stick with a native under your majestic oak.

snowberry gets big fast and does not take much water. It also supplies white berries for the birds.

I planted under an oak this year but I did so very carefully. I had a swale dug and planted drought, dry shade type plants which hopefully will need no water in 2012. Mostly natives like sticky monkey, beauty berry, etc. It looks good, retains the rain water and is dry during the summer.

Penny

Key doc to read from CA Oaks Foundation
http://www.californiaoaks.org/ExtAssets/CompatiblePlantsUnder&AroundOaks.pdf

Here's"Care of California's Native Oaks"
http://www.californiaoaks.org/ExtAssets/oakcaresec.pdf

From Univ of CA
http://ucanr.org/sites/oak_range/Oak_Articles_On_Line/Oaks_and_Landscaping_urban_forestry_/Plant_List_and_Procedures_for_Landscaping_Under_Native_Oaks_of_the_Central_Valley/

Here is a link that might be useful: CA Oaks Foundation