[en] Lattice defects (mainly chalcogen vacancies) drastically affect the optoelectronic properties of monolayer transition metal dichalcogenides (TMDs) grown by chemical vapor deposition (CVD). They can be passivated through charge-transfer doping by laser irradiation in air. Here we perform a systematic in situ study to elucidate the passivation mechanism upon laser irradiation and show a way to controllably n-dope CVD-grown monolayer MoS₂ on SiO₂ substrates. By combining resonance Raman and photoluminescence (PL) spectroscopy we show that an increase in defect density correlates with a redshifted PL emission and hence an increase in electron density. Density functional theory (DFT) calculations identify chalcogen vacancies to be facilitators (not the source) of n-doping, and population of mid-gap levels upon doping lowers the activation barrier for O₂ adsorption from 0.3 to 0.03 eV. Laser irradiation aids in the oxygen-passivation of chalcogen vacancies, manifested by an increase in PL intensity and blueshifted emission, and this blueshift is determined by the laser power density. The passivation occurs on two timescales, with the removal of surface adsorbates first, followed by oxygen adsorption at the sulfur vacancy sites. (paper)