Summary: Molybdenum trioxide (a-MoO.) is a wide bandgap 2D UV.amr layered oxide (Eg-3 eV) that is of interest for resistive switching-based nonvolatile memory devices. It is difficult to produce large-area monolayer MoO through exfoliation of bulk crystals or other techniques. This study demonstrates a facile route to obtain wafer-scale monolayer amorphous MoO, using monolayer 2D MoS, grown by metalorganic chemical vapor deposition (MOCVD) as a starting material, following by UV-ozone oxidate at substrate temperatures as low as 120-C. The process yields smooth, continuous, uniform and stable monolayer oxide with wafer-scale homogeneity. Using the subnanometer MoO as the active layer sandwiched between two metal electrodes, we demonstrate the thinnest oxide-based nonvolatile resistive switching memory with low voltage operation and high ON/OFF ratio. These results, which are potentially extendable to other transition metal oxides, will enable further exploration of subnanometer stoichiometric MoO, extending the frontiers of ultrathin flexible oxide materials and devices. Published in ACS Nano (2022),