Oak trees are found in landscaped areas of both public and private nature, throughout the Tucson and Green Valley areas with mixed success. Usually the species you are seeing is some variety of the Live Oak, Quercus viginiana.
The problem with oak trees at lower elevations, is they struggle to do well, despite what nurseries may tell you. There are many examples of a group of oak trees in both Tucson and Green Valley were half the trees look good, and the other half look terrible. This is the nature of the oak.
Another aspect to keep in mind before buying a live oak is the mature size. Although slow growing, many of this oak reach 50 feet tall and wide; not a small tree by any means. They are too often planted in situations where the full size eventually becomes a problem.
The best shade tree for the desert is still the desert native. Choose either the Velvet Mesquite, Blue Palo Verde, or Desert Willow for the best results.
Reference: Mountain States Wholesale Nursery and Gregg Starr Nursery Plant Info Sheet, 1999.