Transition metal oxides are known for a rich platform to discover emergent properties based on sensitive coupling between charge, spin, orbital, and lattice degrees of freedom. Using first-principles calculations, we demonstrate various emergent properties stabilized in the oxygen-deficient perovskites. First, we predict that tunable ferroelectricity can be realized in oxide perovskites with ordered oxygen vacancies. Specifically, we show that R1/3A2/3FeO2.67 solids (where R is a rare-earth ion and A an alkaline-earth cation) exhibit stable polar phases, with a spontaneous polarization tunable by an appropriate choice of R and A. In addition, Grenier phases with intermediate oxygen deficiencies also play a crucial role in electrochemical systems. We show that the threshold voltage for the reduction of La1–xSrxCoO3 to brownmillerite La1–xSrxCoO2.5 is influenced by the presence of intermediate La1–xSrxCoO2.67 Grenier phases, which are challenging to detect. The Grenier phase creates a barrier for electronic transport and increase the threshold voltage for topotactic phase transitions. Our findings provide insights into the importance of atomic scale control of oxide films for neuromorphic computing applications.