Room-temperature deposition and growth of Au on Si(111) is investigated by means of optical second-harmonic generation. The process on the clean surface is compared under identical conditions to that of the oxygen passivated surface. Ordered Au/Si interfaces occur in both cases. For the surface the interface ordering starts after deposition of 4 monolayers, completing the formation of a phase with some dissolved Si, after which a continuous Au film grows between the substrate and the mixed phase. Oxygen passivation causes interface ordering from a lower Au coverage and a considerably higher degree of interface order. Oscillating second-harmonic generation intensities versus coverage with periods in the 12 - 17 monolayer range show that quantum well states formed in the Au film are responsible for the second-harmonic signal. The annealing behaviours of the Au/Si structures are also studied, and discussed with the inclusion of photoemission results.