Our recent studies on the development of new synthetic methods using molecular oxygen (O2), which is an environmentally friendly oxidant, are described in this Account. The character of O2 as an electron acceptor can be utilized for activation of simple organic molecules to generate reactive species. Such reactive species are applicable to advanced molecular transformation, such as C–C and C–X (X = heteroatom) bond formation, functionalization of inactivated C(sp3)–H, and catalytic Mitsunobu reaction, by avoiding direct quenching of the reactive species by O2.1 Introduction2 Reactions with Iron Catalysts and Oxygen2.1 Reactions Using Redox Hydration of Alkenes2.2 Reactions Using Oxidation of Heteroatoms3 Reactions with tert-Butyl Nitrite and Oxygen4 Conclusion