Oak trees (Quercus spp.) in general are large-growing plants that need ample space to reach their full potential in the landscape. Proper spacing of an ornamental oak is dependent on the specific oak species. Its mature size determines the space required to ensure its roots and branches are not encroached upon by other trees, buildings, or utility lines. Learn the oak's identity and expected spread of canopy at maturity to inform your spacing.
Learn the identity of the oak tree species you are planting. Ask the nursery professional or consult the plant label, looking for both the common and scientific name. The scientific name is vital to the absolute identifcation of the plant as well as finding information specific for the tree.
Consult online or print literature on your oak species, noting the mature height and spread. For example, according to Michael Dirr's book, "Dirr's Hardy Trees and Shrubs", a scarlet oak, Quercus coccineus, grows to a height of 75 feet high and 50 feet wide. Write down or memorize the spread of the tree, here being 50 feet.
Divide the expected mature spread of the oak tree in half. This is the minimum distance it should be planted in proximity to other large trees, power lines, buildings or busy roadways. In the case of the scarlet oak forementioned, it should be planted no closer than 25 feet from a building or other shade tree.
Position the oak so that as it grows to its mature height, it will not infiltrate or obscure important features such as utility lines. This should also be taken into consideration around facilities that need clearance and unimpeded views such as airports or law enforcement and correctional facilities.