In this presentation we discuss the influence of oxide and intermetallide ageing on the structure and mechanical properties of high-alloy structural steels. The principal attention is given to recently created steels strengthened with heat-resistant oxides. Using Mössbauer, electron-microscopy and neutron diffraction methods, analysis was undertaked of the conditions of nanosize oxides introduction in steel and alloy matrices by way of preliminary dissolution of initial large-size oxides in a powder metal matrix (in the course of mechanical alloying in a mill) followed by precipitation of 3-5 nm in size hardening oxides in the process of high-temperature sintering of mechanically alloyed powder. There were obtained record values of high-temperature strength in oxide dispersion strengthened (ODS) steels. The vacancy void formation upon irradiation of stainless steels by high-energy neutrons can be sup pressed by introduction of high densities of direct and indirect point-defect sinks such as α/γ interfaces and fine intermetallic (γ') or oxide particles in fcc or bcc steels precipitated before or upon irradiation. The compositions of new ODS-steels are proposed.