Graphene oxide (GO) has attracted substantial interest for its tunable properties and as a possible intermediate for the bulk manufacture of graphene. GO and its reduced derivatives display electronic and optical properties that depend strongly on their chemical structure, and with proper functionalization, GO can have a desirable bandgap for semiconductor applications. However, its chemical activity leads to a series of unclear chemical changes under ambient conditions, resulting in changes in color and solubility upon exposure to light. In this paper, we study the properties of fresh and spontaneously reduced GO under ambient conditions using tip-enhanced Raman spectroscopy (TERS) to map its nanometer scale chemical and structural heterogeneity. We observe different types of defect sites on reduced GO (rGO) by spatially mapping the D to G band peak ratio and D and G band spectral positions. The higher spatial resolution and out-of-plane polarization compared to conventional micro-Raman spectroscopy enables us to resolve unusual features, including D-band shifting on rGO. Based on statistical analysis of the spatial variations in modes and theoretical calculations for different functional groups, we conclude the reduction mechanism of GO is a self-photocatalytic reduction with the participation of water and visible light, in which the rate determining step is electron transport through the metal substrate and ion diffusion on the GO surface. These results demonstrate that TERS can reveal structural and chemical details elucidating reduction mechanisms, through the examination of samples at different time points.