A fiber-optic dosimetry system using optically stimulated luminescence (OSL) of KBr:Eu was developed and its utility in near-real-time dosimetry has been tested under various medical radiation fields. The OSL response was observed to exhibit component-specific sensitivity changes while operating in the high-dose-rate regions of proton and gamma fields. The present paper provides a more detailed look at the relationship between thermoluminescence (TL) and OSL of KBr:Eu, and proposes a model to describe the mechanisms responsible for the sensitivity change.