[en] Highlights: • Oxide morphology of Fe–15Cr–2W–0.5Ti–7Al–0.4Zr–0.5Y₂O₃ ODS alloy was studied by TEM. • Crystal & interface structures of 134 oxides in it were characterized by HRTEM. • Proportions of various types of oxides were determined according to HRTEM results. • The content ratios of Al/Zr/Ti cause the polymorphic transition of complex oxides. • Mechanisms of thermal and radiation stabilities of the ODS steel were discussed. - Abstract: The nanoparticles in a FeCrAl oxide dispersion strengthened (ODS) steel with Ti and Zr addition, i.e., UOC-10 (Fe–15Cr–2W–0.5Ti–7Al–0.4Zr–0.5Y₂O₃), have been examined by transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). The phases of 134 nanoparticles, which have diameter mainly of 4–11 nm and peak number fraction and, therefore, represent the oxides contributing most significantly to the macroscopic properties of the ODS steel, were identified and, moreover, the proportions of various types of oxides were determined. Relative to FeCrAl-ODS steel without Zr addition, i.e., SOC-9 (Fe–15.5Cr–2W–0.1Ti–4Al–0.35Y₂O₃), the dispersion morphology and coherency of the oxide nanoparticles in UOC-10 were significantly improved. The nanoparticles in UOC-10 were found to be consistent with δ-phase Y₄Zr₃O₁₂ (∼44.8%), defect fluorite Y₂Zr₂O₇ (∼15.7%), perovskite YAlO₃ (YAP) (∼14.2%) and orthorhombic Y₂TiO₅ (∼11.2%) with distorted perovskite YTiO₃ oxide occasionally detected. Almost all of the nanoparticles are either coherent (∼78.4%) or semi-coherent (∼20.3%) with bcc steel matrix. The crystallographic orientation correlations of the oxides and matrix were determined. The results are compared with those of SOC-9 and SOC-14 (Fe–15Cr–2W–0.1Ti–4Al–0.63Zr–0.35Y₂O₃) with a brief discussion of the mechanisms of the formation of various kinds of oxides and, moreover, the unusual thermal and irradiation stabilities of the oxides as well as the excellent irradiation tolerance and superior strength of UOC-10.