This work presents a determination of the photon energy response of Al2O3:C optically stimulated luminescence (OSL) detectors (single crystals and Luxel_ detectors) to photons with mean energies from 10 keV to 1225 keV. Irradiations were performed free-in-air using standard X-ray fields (narrow-spectrum series) specified by the International Organization for Standardization. The OSL readouts were conducted under different conditions (e.g., different optical filters; continuous or pulsed stimulation) and the analyses were performed using the initial OSL intensity or total OSL area. The measured photon energy response of the OSL signal was compared to the ratio between the masseenergy absorption coefficient for Al2O3:C and air. The results demonstrate that the photon energy response of Al2O3:C is not only dependent on the energy deposited, but also on the experimental conditions. This is attributed to the effect of ionization density on the OSL properties of Al2O3:C, which affects the shape of the OSL curve and the relative intensity of the OSL emission bands. The results highlight the importance of maintaining similar OSL readout conditions and using similar types of detectors when determining and applying photon energy correction factors.