The intentional formation of defects in transition-metal dichalcogenides, such as MoS₂, is an attractive way to modify the electronic and chemical properties of this class of 2D materials. However, the mechanisms and methods available for selective doping or modification of the basal plane must be improved. Here we investigate the process of O defect formation in epitaxial single-layer MoS₂ on Au(1 1 1) using scanning tunneling microscopy (STM) and ambient pressure x-ray photoelectron spectroscopy (AP-XPS) during oxidation with O₂ and H₂O gas from low vacuum to the mbar range. Both oxidants result in exchange of S in the upper part of the basal plane with O, in line with air exposure experiments. Temperature-dependent measurements show that this is an activated process with an experimentally estimated reaction barrier of ∼0.79 ± 0.20 eV. We surprisingly find that the morphology of the MoS₂ flakes and their edges remain intact in O₂, even for relatively high degrees of basal plane O exchange, in contrast to the oxidation behavior of exfoliated single-layer MoS₂. From analysis of atom-resolved STM images of the MoS₂ edges, we can attribute this unusual stability to a passivating effect of excess edge sulfur species adsorbed under the sulfiding conditions of the MoS₂ synthesis in H₂S gas. We thus demonstrate that control over pre-sulfidation of the edges, temperature and pressure during oxidation can be used in a fast process to form strongly O doped single-layer MoS₂ with no degradation of the initial shape and edge structure of the epitaxial MoS₂ sheet.