The article discusses issues related to the material aspects of the study of the oxygen isotope effect in High-temperature superconductors. These studies were initiated by Prof. K.A. Müller shortly after he and Dr. G. Bednorz discovered high-temperature superconductivity. Research on the isotope effect was carried out in the years 1991–1995, and then continued for various classes of superconductors and other strongly correlated electron systems. The paper presents methods of synthesis of complex oxide superconductors doped with isotopic oxygen-18. Methods for the determination of oxygen content and isotopic enrichment are also discussed.