The response of Al₂O₃:C optically stimulated luminescence detectors (OSLDs) was investigated in a 250 MeV pencil proton beam. The OSLD response was mapped for a wide range of average dose rates up to 9000 Gys^-1, corresponding to a ~150 kGy s^-1 instantaneous dose rate in each pulse. Two setups for ultra-high dose rates (FLASH) experiments are presented, which enable OSLDs or biological samples to be irradiated in water-filled vials or cylinders. The OSLDs were found to be dose rate independent for all dose rates, with an average deviation <1% relative to the nominal dose for average dose rates of (1 - 1000) Gy s^-1 when irradiated in the setups. A third setup for irradiations in a 9000 Gy s^-1 pencil beam is presented, where OSLDs are distributed in a 3 × 4 grid. Calculations of the signal averaging of the beam over the OSLDs were in agreement with the measured response at 9000 Gy s^-1. Furthermore, a new method was presented to extract the beam spot size of narrow pencil beams, which is in agreement within a standard deviation with results derived from radiochromic films. The Al₂O₃:C OSLDs were found applicable to support radiobiological experiments in proton beams at ultra-high dose rates.