The electronic, catalytic, and optical properties of transition metal dichalcogenides (TMDs) are significantly affected by oxidation, and using oxidation to tune the properties of TMDs has been actively explored. In particular, because transition metal oxides (TMOs) are promising hole injection layers, a TMD–TMO heterostructure can be potentially applied as a p-type semiconductor. However, the oxidation of TMDs has not been clearly elucidated because of the structural instability and the extremely small quantity of oxides formed. Here, we reveal the phases and morphologies of oxides formed on two-dimensional molybdenum disulfide (MoS_​2) using transmission electron microscopy analysis. We find that MoS_​2 starts to oxidize around 400 °C to form orthorhombic-phase molybdenum trioxide (α-MoO_​3) nanosheets. The α-MoO_​3 nanosheets so formed are stacked layer-by-layer on the underlying MoS_​2 via van der Waals interaction and the nanosheets are aligned epitaxially with six possible orientations. Furthermore, the band gap of MoS_​2 is increased from 1.27 to 3.0 eV through oxidation. Our study can be extended to most TMDs to form TMO–TMD heterostructures, which are potentially interesting as p-type transistors, gas sensors, or photocatalysts.