Oxide-surface pits are observable on thick oxides thermally grown on Czochralski-silicon substrates with (111) orientation. The pits were not observed for oxides on [100]-oriented Czochralski-silicon substrates, and they were not observed for oxides on an epitaxial layer on (111)-oriented Czochralski-silicon substrates. For (Ill)-oriented Czochralski-silicon, the number of the pits observed on the oxide increases rapidly with increasing thickness of the oxide grown thermally in dry O-2 ambient at higher temperatures. We observed silicon-surface dents just under the oxide-surface pits. The shape of the dents was triangular and that of the pits was circular. A model is presented which shows how octahedral void defects on the silicon surface lead to the formation of oxide-surface pits during high temperature oxidation.