This work investigated the high-temperature oxidation kinetics and evolution of oxide phases of as-fabricated (AF) and heat-treated (HT) Inconel 718 alloys in air fabricated by the wire-arc additive manufacturing process. The results showed that the oxidation kinetics of all samples followed the parabolic time dependence. The microstructural study revealed the formation of very similar oxide scales in both AF and HT conditions. The oxide surface was found to be a mixture of complex spinel-MnCr2O4 and rutile-TiO2 phases. The cross-sectional view yielded an outer layer of protective Cr2O3 scale, an inner layer of Nb-rich Ti0.67Nb1.33O4 oxide and discretely formed internal Al2O3 oxides. The mechanism of scale formation is suggested on the basis of thermodynamic and kinetic abilities of cationic species.