Spatially distributed DNA oligomer arrays on Au(111) surfaces were created by one-step co-assembly of mixed monolayers of alkanethiol-conjugated DNA and mercaptohexanol (MCH). Tapping-mode AFM was used to visualize the distribution of DNA molecules on the surface and to study the mechanical properties of individual molecules. The DNA coating density increased nonlinearly with increasing mole fraction of DNA oligomer to MCH in the coating solution. For imaging in air, where surfaces are coated with a thin layer of water, the interaction between the AFM tip and the different structures on the monolayer varies between attractive and repulsive depending on the tapping amplitude, set-point ratio, and tip shape. It was found that both duplex and single-stranded DNA molecules extend approximately vertically upward from the surface.