<p>Oxide dispersion-strengthened steel PM2000 in the recrystallized condition was subjected to 3.5&#xa0;MeV Fe¹³⁺ ion irradiation at room temperature (RT) and at 400°C to 0.54 dpa and 0.52 dpa, respectively. The dispersoids of the steel before and after irradiation were investigated using transmission electron microscopy, high-resolution transmission electron microscopy, and energy-dispersive spectroscopy. Several types of oxide phases, YAlO₃, Al₂Y₄O₉, and Y₃Al₅O₁₂ with simple tetragonal and b.c.c. lattices and YCrO₃ and Al₂O₃ with hexagonal and simple orthorhombic lattices, were identified in the unirradiated steel. Irradiation resulted in a significant increase in the number of fine dispersoids in the steel, which increased with increasing irradiation temperature. YAlO₃ and Y₃Al₅O₁₂ with a simple tetragonal lattice and Al₂O₃ with a hexagonal lattice were stable in their nature under the two irradiation conditions. Y₃Al₅O₁₂ with a b.c.c. lattice was likely dissolved under irradiation at RT. Y₃Al₅O₁₂ with a b.c.c. lattice was considered to be dissolved, while YCrO₃ and Al₂Y₄O₉ might be dissolved under irradiation at 400°C. Irradiation at 400°C induced the formation of ultrafine-/nanoscale Y₂O₃ dispersoids with a base-centered monoclinic lattice throughout the steel matrix. The formation of an irradiation-induced Y₂O₃ phase is also discussed.</p>