An air-promoted radical addition reaction of various thiols to oxa(aza)bicyclic alkenes is reported, leading to the corresponding exo adducts in good to excellent yields (up to 99%) in water without the use of any metal catalyst, photoredox catalyst or radical initiator reagent under mild conditions. The reaction has a wide scopeof oxa(aza)bicyclic alkenes and thiols, which provides an easy and clean way for the direct preparation of thioethers. A proposed mechanism of an exo free radical attacking the double bond of an oxa(aza)bicyclic alkene was investigated by DFT calculations of stepwise reaction pathways and control experiments.