Near-stoichiometric amounts of 18O2 and 17O2 were generated ex situ from endoperoxides in a two-chamber glassware to oxidize various substrates. This strategy gave [*O2]endoperoxides, [*O1]quinones, [*O1]phenols, and [*Ox]artemisin in moderate to good yields and high isotopic enrichments (up to 84%) at affordable costs. Moreover, mass spectrometry and 17O NMR of the [*O]products provided valuable information about the chemical mechanisms involved.