The early stages of thermal oxidation of the basal plane of highly oriented graphite (HOPG) have been investigated in ozonized air. Scanning tunneling microscopy (STM) was employed to determine the surface topography on the nanometer scale. At temperatures within the range 300-500 degreesC and at O-3 concentrations below 1 ppm, the abstraction of carbon atoms from the basal plane, that is, formation of pointlike defects, occurs. These defects upon further oxidation are enlarged to monolayer deep pits with diameters in the range 1-100 nm. Pit formation does not proceed continuously during oxidation and is strongly inhibited when a critical pit density is reached. The pits are not generated very close to the pit edges with the consequence of a quite uniform not purely statistical coverage of the HOPG surface with the pits. Oxidation time, temperature, and O-3 concentration can be used to control the pit density and their distribution across the surface, the pit diameter, and the diameter distribution with unprecedented reproducibility. Well-defined nanostructured HOPG surfaces with a pit density in the range 10-25.000 mum(-2) can be obtained.