The formation of interfacial metal-oxide structures on the Pt3Ni(111) bimetallic surface was investigated using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) techniques at near-ambient pressure (NAP). Direct observation of surface images clearly shows the occurrence of surface segregation of the sub-surface Ni depending on the surrounding gas-phase conditions. Especially, the prepared topmost Pt-skin layer of the Pt3Ni(111) is altered by Ni oxide segregation that makes an interfacial Pt-NiO1-x nanostructure with dissociated oxygen. This metal-oxide interface could provide active sites for more-efficient carbon monoxide (CO) conversion processes under mixed CO/O-2 gas environments; the associated specific chemical binding energy was identified using NAP-XPS. The combined operando observations from the NAP-STM and NAP-XPS on the Pt3Ni(111) surface reveal that the interfacial metal-oxide structure is strongly correlated with the origin of the enhanced catalytic activity at thermodynamic equilibrium.