The effect of oxygen on adhesion and chemistry at interfaces between Cu thin films and SiNx barrier layers on Si substrates was investigated. Films were deposited in an ultra-high-vacuum sputter deposition chamber with good control of oxygen content. Adhesion was measured using a high throughput driver film method. Microstructure and bonding were investigated using transmission electron microscopy and electron energy loss spectroscopy, respectively, on sample cross sections. For films to which a small amount of oxygen was added during deposition, oxygen segregated to Cu/SiNx interfaces during thermal cycling, where it induced charge transfer and the presence of Cu+ interfacial states, and significantly reduced the work of adhesion.