Au-Cu bimetallic nanoparticles (NPs) grown on TiO2(110) have been followed in situ using grazing incidence X-ray diffraction and X-ray photoemission spectroscopy from their synthesis to their exposure to a CO/O2 mixture at low pressure (P < 10-5 mbar) and at different temperatures (300 K-470 K). As-prepared samples are composed of two types of alloyed NPs: randomly oriented and epitaxial NPs. Whereas the introduction of CO has no effect on the structure of the NPs, an O2 introduction triggers a Cu surface segregation phenomenon resulting in the formation of a Cu2O shell reducible by annealing the sample over 430 K. A selective re-orientation of the nanoparticles, induced by the exposure to a CO/O2 mixture, is observed where the randomly oriented NPs take advantage of the mobility induced by the Cu segregation to re-orient their Au-rich core relatively to the TiO2(110) substrate following specifically the orientation ((111)NPs//(110)TiO2) when others epitaxial relationships were observed on the as-prepared sample.