The radiation response of a phosphorus-doped multimode optical fiber is investigated under both transient (pulsed X-rays) and steady-state (γ - and X-rays) irradiations. The influence of a H2 preloading on the fiber radiation-induced attenuation (RIA) in the 300-2000-nm wavelength range has been characterized. To better understand the impact of this treatment, online behaviors of fiber samples containing different amounts of gas are compared from glass saturation (100%) to less than 1%. In addition to these in situ experiments, additional postirradiation spectroscopic techniques have been performed such as electron paramagnetic resonance or luminescence measurements to identify the different point defects responsible for the induced losses and their H2 dependence. All our data at room temperature (RT) highlight a strong positive impact of H2, even at very low concentrations, on the RIA. Hydrogen quickly passivates (t < 1 s) most of the defects responsible for the visible-near-IR RIA, mainly phosphorus oxygen hole centers (POHC) and P1 defects. However, 1 year after the H2 loading at RT or when operating at liquid nitrogen temperature, the RIA levels of the not-treated and H2-loaded fiber become comparable. The obtained results provide a better understanding of the potential and limitations of H2-loading treatment to design radiation-hardened fiber links.