An optical in situ strategy for the analysis of oxygen diffusion in ultrathin ceria layers with a thickness of 2-10 nm at temperatures between 50 and 200 °C is presented, which allows for the determination of diffusion coefficients. This method is based on the sensitivity of the photoluminescence (PL) intensity of InGaN nanowires to adsorbed oxygen. The oxygen diffusion through an ultrathin CeO coating deposited on the InGaN nanowires is monitored by analyzing the transient PL behavior of the InGaN nanowires, which responds to changes of the oxygen concentration in the environment when the corresponding oxygen concentration is established at the CeO/InGaN interface due to diffusion through the coating. Quantitative evaluation of the oxygen diffusion in CeO based on a model considering Langmuir Adsorption and recombination yields a diffusion coefficient D of (2.55 ± 0.05) × 10 cm s at a temperature of 100 °C. Temperature-dependent measurements reveal an Arrhenius type behavior of D with an activation energy of (0.28 ± 0.04) eV. In contrast, no oxygen diffusion is detected for an ultrathin layer (≥5 nm) of AlO, which is known as a poor oxygen ion conductor within the analyzed temperature regime.