Oxide-dispersion strengthened steels characterization using various spectroscopic techniques is presented. Microstructure of 15% chromium oxide-dispersion strengthened steels was studied in term of vacancy defects presence and their accumulation after defined irradiation treatment, respectively. Studied materials originated from Kyoto University and studied via IAEA collaborative project focused on generation IV reactors (ALLEGRO). Samples were characterized "as received" by positron annihilation lifetime spectroscopy, the Mössbauer spectroscopy and their microstructure was examined by transmission electron microscopy as well. Samples were afterwards irradiated in Washington State University Nuclear Radiation Center via a strong gamma source (6 TBq). Damage induced by gamma irradiation was evaluated by positron lifetime measurements in emphasis on defect accumulation in the materials. We have demonstrated strong defect production induced by gamma irradiation which results from positron measurement data.