“Warm Soil Cool Air” sums up the philosophy of those who believe as I do that fall is the best time for planting olive trees. Planting a tree in early fall after the intense heat of the summer has passed relieves the tree of suffering through a period of high stress that heat can bring on and which often results in setting the tree back and retarding growth. This is also a philosophy shared by my friends in the olive nursery business in Italy.
As the soil is still relatively warm the roots are stimulated to grow while the foliage part of the plant enjoys the mildness provided by short days and nights not yet cold. Good root development first lets the tree get ready for abundant spring growth.
While it is true that a younger tree is more sensitive to extremes of cold, planting in early October gives the tree two months to harden up before the coldest period of the year. Also as much of California is quite mild in winter (more so than much of central and northern Italy) cold is often not the primary consideration of when to plant.
If for whatever reason you wish to plant at another time of year, early spring is a good choice. Again, subjecting the tree to as little stress is the objective. Early spring avoids the hard frosts of December and January as well as the extremes of summer.
Given what seems to have become the real unpredictability of the rainy season in California, drip irrigation is a must no matter when one chooses to plant. By using drip those in the milder areas have had success planting during most of the year. Early fall however remains optimal.