We have studied the interaction of oxygen with Au overlayers on Ru(100) by examining Au deposition on oxygen-precovered Ru and comparing this with oxygen adsorption on Au-precovered Ru(100). Au deposition on oxygen-precovered Ru(100) led to an increase in the local atomic density of the oxygen phase and enhanced 3-dimensional growth in the Au overlayer relative to Au deposition on the clean substrate, though with increasing Au coverage the preadsorbed oxygen was covered with Au. Annealing of these overlayers resulted in an extensive agglomeration of the Au with the oxygen in intimate contact with the Ru such that part of the surface was covered only with oxygen. The Au rewets the Ru on desorption of the oxygen, which is completed, in the presence of > 3 ML Au, before the completion of Au desorption, i.e., at some 300 K lower temperature than in the absence of Au.