Titanium nitride (TiN) is widely used in industry as a protective coating due to its hardness and resistance to corrosion and can spontaneously form a thin oxide layer when it is exposed to air, which could modify the properties of the coating. With limited understanding of the TiO2 – TiN interfacial system at present, this work aims to describe the structural and electronic properties of oxidized TiN based on a density functional theory (DFT) study of the rutile TiO2 (110) – TiN (100) interface model system, also including Hubbard +U correction on Ti 3d states. The small lattice mismatch gives a good stability to the TiO2 – TiN interface after depositing the oxide onto TiN through the formation of interfacial Ti – O bonds. Our DFT+U study shows the presence of Ti3+ cations in the TiO2 region, which are preferentially located next to the interface region as well as the rotation of the rutile TiO2 octahedra in the interface structure. Although the vacancy formation energies for Ti in TiN (Evac (Ti) = 4.03 eV) or O in the oxide (Evac (O) = 3.40 eV) are quite high relative to perfect TiO2 – TiN, defects are known to form during the oxide growth and can therefore be present after TiO2 formation. Our results show that a structure with exchanged O and N can lie 0.82 eV higher in energy than the perfect system, suggesting the stability of structures with interdiffused O and N anions at ambient conditions. The presence of N in TiO2 introduces N 2p states localized between the top edge of the O 2p valence states and the mid-gap Ti3+ 3d states, thus reducing the bandgap in the TiO2 region for the exchanged O/N interface EDOS. The outcomes of these simulations give us a most comprehensive insight on the atomic level structure and the electronic properties of oxidised TiN surfaces.