The effect of post-oxidation annealing in Ar atmosphere (Ar POA) on 4H-SiC–oxide interfaces has been studied by capacitance to gate-bias voltage (C–V) measurements and photoemission spectroscopy (PES). It was found from the C–V measurements that the shift of the C–V curve disappears when the Ar POA temperature is higher than 600 °C. On the other hand, angle-resolved x-ray photoelectron spectroscopy measurements revealed that the thickness of the intermediate layers containing Si1+ oxidation states observed at the interfaces decreases abruptly when the Ar POA temperature exceeds 500 °C. In ultraviolet photoelectron spectra, O2p peaks were changed by Ar POA at temperatures higher than 600 °C, which is the temperature where the shift of the C–V curve disappears in C–V measurements. This shows that the change in O2p bonding by Ar POA is the origin of the shift observed in C–V characteristics. A model of structural changes in the interfaces by Ar POA has been proposed based on the results of PES measurements and those of C–V measurements.