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Several experimental groups have shown that defect structures in 2D WSe₂ result in single photon emission (SPE). However, the origin of SPE is still unknown. We present a first principles study of the nature and optical properties of point defects in 2D WSe₂, together with scanning tunneling microscopy (STM) and scanning transmission electron microscopy images. We predict that O₂ can dissociate easily at Se vacancies, resulting in O-passivated Se vacancies (O_Se) and O interstitials (O_ins), which give STM images in good agreement with experiment. Our GW-Bethe-Salpeter-equation calculations show that Oins defects give exciton peaks ~50-100 meV below the free exciton peak, in good agreement with the localized excitons observed in independent SPE experiments. No other point defect (O_Se, Se vacancies, W vacancies, and SeW antisite defects) gives excitons in the same energy range. We conclude that the Oins defect is a source for the SPE previously observed in 2D WSe₂.